Thanks @gr5! That makes a lot of sense, and is a very clever solution.
The hassle that I'm having now is that the AA nozzle is pushing a little too far on the rubber nozzle protector thing on the fan cover and is preventing it from closing properly.
I deduced that this is the problem because putting the AA core into the BB slot and flipping that switch allows me to clsoe the fan cover nicely (with the BB core now out of the machine entirely).
The image below (which has both AA and BB cores installed) hopefully shows you what I mean
Thanks for taking the time to help!!!