Yes it can be tapped to fit 12mm AV switches. You need to know the thread pitch of the switch you choose to install. To do it you have to attach the wires first then turn the switch in while managing the wires inside. It is not an easy thing and takes a good amount of patience but I like the look and like doing it to mine.
I'm very interested in doing just this as well (ie change out guarded to an AV).
A few questions.
1. What do you mean by "You need to know the thread pitch of the switch you choose to install?"
2. After getting the guarded out then the objective is to drill a 12mm hole for the new switch?
3. Will any 12mm AV switch work or are there different varients?
Thanks for helping!