Win10 doesn't recognize Start8 as incompatible, and upgrading to Win10 with Start8 still installed yielded no issues. It works as it always had.
In fact, it wasn't until I enabled beta in desktop manager that I noticed Start10 was there. I'm hesitant to switch, as Start8 still provides exactly what I want.
Well done, Start8 team. It's great to see such a solid piece of work!