The comments in this discussion are not useful unless backed by reference to legislation of some kind.
I can't find anything in the act referring to change of address, but my licence condition #3 states 'without undue delay' inform the chief officer of police of a change of address. I also can't find a definition of 'undue delay' with respect to firearms licencing, so I guess that's a point you could argue given individual circumstances. The 'chief officer' is for the force in the 'area where you reside', so that could be either depending on if you inform them before or after your move.
So I'd say, no offence on the first two counts, but unless you have a good reason for the delay in notifying them of your new address, you could be charged with an offence for not meeting the conditions of your licence.