I'm comparing GroovyArcade-Arch2013.02.24-x86_64 with GroovyArcade-Arch2015.02.15-x86_64.
I have managed to get Wah!cade running by doing exactly what I said earlier.
1. Made an install of the 2015 beta release, copied it to my HDD and set it up with EFI booting, etc. This works very well.
2. Copied this installation and the EFI boot files to a USB stick for safe keeping then deleted all the files on my HDD.
3. Made an install of the stable release and copied it to my HDD (I couldn't actually get this one working on my system).
4. Copied all the files from the first installation on the USB stick back to the HDD overwriting the stable installation.
I haven't had much time to play with it but Wah!cade boots and exits properly.
I have no idea how much dead crap is left on my PC from the first install but it probably doesn't matter.