Apps should work. I think Microsoft would want to avoid the backlash if they stopped supporting apps for Windows 8 and Windows 8.1 when the apps are pretty much the same in 10. If you have the upgrade icon in the lower right hand corner, I do recommend reserving your free copy of 10, even though you may not want to migrate right away. I have been running the 10 technical preview for a while now and it has some much needed improvements over Windows 8. Windows 8 and 8.1 should work exactly the way you purchased, you just may have that pesky upgrade icon if you do not reserve your copy.
After the July update of Windows 10, If 10 is not installed or reserved. Will 8.1 stay the same ? Thanks Chi Ed.
What will happen to Windows 8.1 ?