Attached is a Lua script which allows switching between two CHDK builds. Autoboot builds can only be switched on dryos, manual boot files on any camera. Read all the usage and warnings in the script comment before running it.
I have only tested this very briefly on a570, which is a vxworks camera (PS.FIR swapping only tested).
It wouldn't be too hard to make the script able to switch between an arbitrary number of builds, but I don't have much use for that as long as DISKBOOT.BIN remains hidden in vxworks.