it varies... some planes never seem to change, even if you made them different... i'd say there are a handful of planes this applies to, beats me if i can name them all... i recall the spit9 being one to change now and then... and probably one of the 109's iirc. there's almost no consistency or pattern behind it. never hurts to check ur 1st ride(s) in a plane(s) of the day after u log in.
easiest way to totally prevent this from happening is to go thru all the cfg files (planetype.cfg) and manually set your convergences - making sure to uncheck the read-only attribute before editing it. then setting your convergences (in feet) then saving and then re-checking the read-only. the only difficult thing can be determining which lin ein teh cfg file applies to which set of guns. I always have all guns on a plane set to converge the same distance so it's no big for me to have done that. some people, however, like to vary their convergence settings, i.e. many fweenies like to do that, apparently, and that would require more effort in editing the files.
then, you can still change convergence in game, but next time you open AH it'll default back to the saved .cfg file settings.
it's been a long time since i've been hit by that, so i'm unsure if in fact perhaps HT had fixed it whenever. which is why i was suggesting to check, just in case it's still present.