If you upgrade from the 8 RTM to the 8.1 RTM, nothing will be removed. But if you use the Preview (even if you rollback later) then all Modern UI apps and all Desktop apps have to be reinstalled.
I'll do something else: First of all I'll clone my running Windows8 on a VHDX (Virtual Harddisk, supports TRIM) from Paragon12. So I keep my volume serial and Windows8 stays activated. Then the VHDX will be copied on my SSD root and then I'll create the BCD entry to boot from it. With this clone I'll upgrade to Windows8.1 to see which apps are kept or not & what works. Anyone knows - will windows stay actived or do we get the "Developer Preview" notes on Desktop?
If you install by ISO, you will need to enter the win8WRP key ****B7QPF(the key will be posted in the MS preview page), you can activate it then against MS servers, actually if you are online, it will probably got activated by itself during install. btw, Preview only comes in Pro, that's why if you are on win8 E, you couldn't upgrade it to Preview and have to install by ISO.
Instead of cloning or dual booting, you can create a refresh image with recimg.exe (recimg.com if you want a nice GUI), upgrade to 8.1 Preview and wait for RTM then use your custom refresh, voila back to 8.0 and now upgrade to 8.1, all while staying on your current Install, then again.. if 8.0 > 8.1 Preview doesn't break anything, im sure you could as well just set your Version back to 8.0 (registry) when RTM hits and install 8.1 Final without losing anything, but who knows what might break, using a VHD is still the best way though.
People...don't complain here pls about that link..just few hours and we'll have that Windows and I can bet soon we'll be bored with it.