Ok, I *think* that I got this correct.
This swaps the slots so you'll be on _a
You should try with everything back to as it was.
You can try various experiments that you already did to see if something is different.
Make sure that you have the custom recovery on _a and _b
Code:
edl /w /s0 /c34 main.img
edl /x /s-33 /c33 backup.img
Make sure that you have the whole big raw backup in case of major screwup.