Upgrade Fusion>SSD--when to break fusion link?

Hello!

I'm planning to crack open my mac and replace the HDD part of my current Fusion setup (but leave the blade SSD in place). I think I understand what I need to do from physical standpoint, and I understand how to break the Fusion drive in Terminal. Thanks for all the great advice on that in these forums.

What I'm not so sure about is when I need to actually break the Fusion link in Terminal. Would I do that before or after I make the physical switch of the drives?

Thanks in advance!