Is it possible and are there any rules for, swapping specialisations on abilities.
Consider. Bob the apprentice has an Artes Liberales score of 2 with a specialisation in Grammar. Over the course of his Hermetic lifetime he learns the celestial magic chain and devotes his studies to Astronomy plowing loads of xp into Artes Liberales and putting it up to 9. However, his speciality is still grammar. Could he change this to something more appropriate such as astronomy? If so, how often?
Since i can't find any rules about this, how about allowing Players to swap out specialisations whenever they level the ability up? That limits it to when the magus has actually been studying and stops it happening too often.
We've used that rule (swapping specializations when the ability levels up) in my sagas for a while and it has worked well for us. It gives a good sense of character growth and allows players to fine tune their characters to better fit the realities of the saga.
I read a summa or tractatus on a different subject which is more interesting than the previous (for exemple, my ex miscelenea had philosophiae meta physics and is reading a summa in philosophy in ritual, so I switch to ritual magic... and I know later I will switch to alchemy with her
for a language when I spend enought time speaking in that specialisation (if my ex miscelenea had at first her latin specialised in "religious" as she was raised in a religious monastery, after a few years in the covenant, she has switch naturally to hermetic)
if I practice in a speciality I may change (If I first learned single weapon with long sword and later i practice more with the whip, I may switch to whip)
I agree with the fact that it is better to switch when you get a new level, otherwise you may spend a season during which you don't gain experience point but switch your speciaity (it is what I did when, in nature lore, I switch forest, but it is different as there is no real specialisation in nature lore)
We also allow a specialization change when - and only when - the score increases. This seems to be one of the most widely used house rules We also require that the change must have an in-character justification, but I've never seen anyone have problems with finding one.