Responding from my new Windows 10 system after an about 20 minute upgrade. Unfortunately, it took out the Explorer backup on the go (even the one in Windows.old), so gonna have to recreate it.
Anyway, to clarify, I'm talking about the video and image thumbnail preview sizes in Explorer (the default choices being Extra large icons, Large icons, Medium icons, etc.) - that is, whether it will be the same thumbcache_*.db both are saved into.
If you have all of them in the same preview sizes, then that's just weird.