You cannot use Coffee Lake CPU's (as in what the 2019 uses) on the 2017 for two (very) major reasons:
(1) The 2017 uses an LGA 1151 rev 1 socket, and the 2019 has the LGA 1151 rev 2.
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(2) The chipsets on each logic board are (ver) different: z170 vs z370
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(2) The chipsets on each logic board are ***very different***: z170 vs z370
LGA 11551 rev1 vs rev2: Not same sockets: the pin allocations are different [link|https://en.wikipedia.org/wiki/LGA_1151#Kaby_Lake_chipsets|Second revision of the LGA 1151 socket for Coffee Lake CPUs]:
"While physical dimensions remain unchanged, the updated socket reassigns some reserved pins, adding power and ground lines to support the requirements of 6-core and 8-core CPUs. The new socket also relocates the processor detection pin, breaking compatibility with earlier processors and motherboards. As a result, desktop [link|https://en.wikipedia.org/wiki/Coffee_Lake|Coffee Lake] CPUs are officially not compatible with the 100 (original Skylake) and 200 (Kaby Lake) series chipsets."
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Edit:
In fact, there's a VERY detailed comparison of Kirby Lake/Coffee Lake pinouts (i.e. on the interface betwwen CPU and socket). And it explains the very big differences between the two. It even says a firmware upgrade would not be able to address it. The only (remote) possibility would be on some motherboards a z270 chipset is used, whicxh is closer to the z370. But iMac went from z170 to z370 directly.
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[link|https://www.eteknix.com/kabylake-vs-coffee-lake-lga1151-pinout-differences-revealed/|Kabylake vs Coffee Lake LGA1151 Pinout Differences Revealed|new_window=true]
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[link|https://www.eteknix.com/kabylake-vs-coffee-lake-lga1151-pinout-differences-revealed/|Kabylake vs Coffee Lake LGA1151 Pinout Differences Revealed]
Anadtech discusses it too:
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[link|https://www.anandtech.com/show/11859/the-anandtech-coffee-lake-review-8700k-and-8400-initial-numbers/3|Differences from Coffee Lake to Kaby Lake Physical Design: Pin Outs|new_window=true]
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[link|https://www.anandtech.com/show/11859/the-anandtech-coffee-lake-review-8700k-and-8400-initial-numbers/3|Differences from Coffee Lake to Kaby Lake Physical Design: Pin Outs]
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Chipsets: The 2017 logicboard has the z170 chipset, whereas the 2019 has the z370.
Here's a nice link on the differences between the chipsets (look under LGA 1151 rev1 and rev2).
[link|https://en.wikipedia.org/wiki/List_of_Intel_chipsets|List of Intel Chipsets]
The z170 is quite different from the z370 in terms of what it supports.
And note that there was an intermediate z270 chipset in-between them, that is very close to the z370 (specs are very close here).
Intel says that the z170 cannot support the Coffee Lake CPU's (used in 2019 iMac):
Can see that the z170 is not forwards compatible, and the z370 not backwards compatible, and even the z270 won't support Coffee Lake CPU's.
And this is part speculation on my end, but I think the EFI chips/firmware difference is not really an issue. You can both back-up and re-flash these chips on the board. And re-soldering one is no big deal: are simple chips, and you can buy them.
You cannot use Coffee Lake CPU's (as in what the 2019 uses) on the 2017 for two (very) major reasons:
(1) The 2017 uses an LGA 1151 rev 1 socket, and the 2019 has the LGA 1151 rev 2.
(2) The chipsets on each logic board are (ver) different: z170 vs z370
-
LGA 11551 rev1 vs rev2: Not same sockets: the pin allocations are different [link|https://en.wikipedia.org/wiki/LGA_1151#Kaby_Lake_chipsets|Second revision of the LGA 1151 socket for Coffee Lake CPUs|new_window=true]:
+
LGA 11551 rev1 vs rev2: Not same sockets: the pin allocations are different [link|https://en.wikipedia.org/wiki/LGA_1151#Kaby_Lake_chipsets|Second revision of the LGA 1151 socket for Coffee Lake CPUs]:
-
"While physical dimensions remain unchanged, the updated socket reassigns some reserved pins, adding power and ground lines to support the requirements of 6-core and 8-core CPUs. The new socket also relocates the processor detection pin, breaking compatibility with earlier processors and motherboards. As a result, desktop [link|https://en.wikipedia.org/wiki/Coffee_Lake|Coffee Lake|new_window=true] CPUs are officially not compatible with the 100 (original Skylake) and 200 (Kaby Lake) series chipsets."
+
"While physical dimensions remain unchanged, the updated socket reassigns some reserved pins, adding power and ground lines to support the requirements of 6-core and 8-core CPUs. The new socket also relocates the processor detection pin, breaking compatibility with earlier processors and motherboards. As a result, desktop [link|https://en.wikipedia.org/wiki/Coffee_Lake|Coffee Lake] CPUs are officially not compatible with the 100 (original Skylake) and 200 (Kaby Lake) series chipsets."
+
+
----------------------------------------
+
+
Edit:
+
+
In fact, there's a VERY detailed comparison of Kirby Lake/Coffee Lake pinouts (i.e. on the interface betwwen CPU and socket). And it explains the very big differences between the two. It even says a firmware upgrade would not be able to address it. The only (remote) possibility would be on some motherboards a z270 chipset is used, whicxh is closer to the z370. But iMac went from z170 to z370 directly.
+
+
[link|https://www.eteknix.com/kabylake-vs-coffee-lake-lga1151-pinout-differences-revealed/|Kabylake vs Coffee Lake LGA1151 Pinout Differences Revealed|new_window=true]
+
+
Anadtech discusses it too:
+
+
[link|https://www.anandtech.com/show/11859/the-anandtech-coffee-lake-review-8700k-and-8400-initial-numbers/3|Differences from Coffee Lake to Kaby Lake Physical Design: Pin Outs|new_window=true]
+
+
------------------------------------------
Chipsets: The 2017 logicboard has the z170 chipset, whereas the 2019 has the z370.
Here's a nice link on the differences between the chipsets (look under LGA 1151 rev1 and rev2).
-
[link|https://en.wikipedia.org/wiki/List_of_Intel_chipsets|List of Intel Chipsets|new_window=true]
+
[link|https://en.wikipedia.org/wiki/List_of_Intel_chipsets|List of Intel Chipsets]
The z170 is quite different from the z370 in terms of what it supports.
And note that there was an intermediate z270 chipset in-between them, that is very close to the z370 (specs are very close here).
Intel says that the z170 cannot support the Coffee Lake CPU's (used in 2019 iMac):
Can see that the z170 is not forwards compatible, and the z370 not backwards compatible, and even the z270 won't support Coffee Lake CPU's.
And this is part speculation on my end, but I think the EFI chips/firmware difference is not really an issue. You can both back-up and re-flash these chips on the board. And re-soldering one is no big deal: are simple chips, and you can buy them.
You cannot use Coffee Lake CPU's (as in what the 2019 uses) on the 2017 for two (very) major reasons:
(1) The 2017 uses an LGA 1151 rev 1 socket, and the 2019 has the LGA 1151 rev 2.
(2) The chipsets on each logic board are (ver) different: z170 vs z370
LGA 11551 rev1 vs rev2: Not same sockets: the pin allocations are different [link|https://en.wikipedia.org/wiki/LGA_1151#Kaby_Lake_chipsets|Second revision of the LGA 1151 socket for Coffee Lake CPUs|new_window=true]:
"While physical dimensions remain unchanged, the updated socket reassigns some reserved pins, adding power and ground lines to support the requirements of 6-core and 8-core CPUs. The new socket also relocates the processor detection pin, breaking compatibility with earlier processors and motherboards. As a result, desktop [link|https://en.wikipedia.org/wiki/Coffee_Lake|Coffee Lake|new_window=true] CPUs are officially not compatible with the 100 (original Skylake) and 200 (Kaby Lake) series chipsets."
Chipsets: The 2017 logicboard has the z170 chipset, whereas the 2019 has the z370.
Here's a nice link on the differences between the chipsets (look under LGA 1151 rev1 and rev2).
[link|https://en.wikipedia.org/wiki/List_of_Intel_chipsets|List of Intel Chipsets|new_window=true]
The z170 is quite different from the z370 in terms of what it supports.
And note that there was an intermediate z270 chipset in-between them, that is very close to the z370 (specs are very close here).
Intel says that the z170 cannot support the Coffee Lake CPU's (used in 2019 iMac):
[link|https://en.wikipedia.org/wiki/LGA_1151#Kaby_Lake_chipsets|LGA 1151|new_window=true]
Look under the LGA 1151 rev1 and rev2 tables.
Can see that the z170 is not forwards compatible, and the z370 not backwards compatible, and even the z270 won't support Coffee Lake CPU's.
And this is part speculation on my end, but I think the EFI chips/firmware difference is not really an issue. You can both back-up and re-flash these chips on the board. And re-soldering one is no big deal: are simple chips, and you can buy them.