I would really find it odd if they'd do that with battletomes...I'd expect them to put it either in a generals handbook or possibly a wizards handbook of sorts. If you start splitting coremechanics like magic across multiple battletomes people will be confused as hell. Not to mention that you then run into severe issues with 1 faction having an old outdated tome. Do they wait all the way till a new tome comes out before they get a magic update? Do they just randomly get a new tome that only really updates a paragraph cuz it updates the base spells? What happens if you face one with a much newer battletome and both are magic heavy armies? It'd be a disaster...
Admittadly, that's not to say they can't put minor changes in the various battletomes, but an overhaul of the basic-spells or other core aspects of magic would need to be in a general-purpose, or magic-specific book. Not in a battletome....
As for
@pendrake 's idea... yea it'd be nice for magic to do a bit more. Currently it seems to boils down to A) Mortal wounds B) A minor buff (+1 to save, re-rolls) C) Summon reinforcements.... There's some minor variations in the attacking spells, some work better against hordes, some better against cowardly units etc, but these difference could be made much greater. Hell give us spells that deal regular wounds if that means we can actually make a dedicated wizard to bombard large formations or one to snipe the powerfull monsters.... similarly spells that change the terrain, summon hazards, spells that last more than a single turn, spells that add a mechanic (e.g. a spell that invokes terror making a unit liable to ignore orders and flee).. there's so much more that can be done with magic..