Actually I couldn't keep my paws off this problem and created a pretty simple solution (it was easier than I thought thanks to remembering that the variable is only modified in the local prompt and not globally).
Paste this in a text file:
set appdata=C:\Minecraft <--- change this to whatever directory you want the Minecraft data files to be stored in
Name the text file something like "Minecraft updated.bat" or something, the only important thing is to change the .txt to .bat as file ending.
Then just place the bat-file in the same director you put Minecraft.exe in.
If you want to start the newest version of Minecraft you start Mineraft updated.bat, if you want to start the version you can play with on TOG servers you start the normal Minecraft.exe.
A fun thing you can do if you have several computers connected on a network is to set the path to a network share and thus the clients on all computers will get updated at the same time, and they will share the save files!