Ion screen vs Sapphire screen?

You may want to rethink this!

While it's technically possible you will loose the water tightness a factory installed screen has. You will also have less visibility to the screen as Sapphire is not as transparent as the Ion glass screen.

Lastly, while the Ion glass can scratch a bit more easily, the Sapphire can fracture if struct with a sharp object (brittle) unlike the Ion glass. Here's a more:

• Sapphire versus Mineral Watch Crystal
• Does Gorilla Glass 5 mean the end for sapphire screens

I would stick with the Ion glass my self if you need to replace it otherwise leave it alone.

Here's the newest version of Gorilla glass: Corning - Gorilla Glass SR and here's another writeup from Engadget: Corning's latest Gorilla Glass will make wearables tougher - Gorilla Glass SR

With this announcement the need for Sapphire for coverglass in either phones or watches is now almost pointless as the differences is so little!