One would assume that with the design choice of 8 abilities the mages were rebuild from the ground up to work with this system. And BioWare even confirmed before that no ability from the past games was guaranteed to be in DAI as they looked at every ability and basically rebuild the classes.
This is why I'm not too worried about the change.
Like if Inquisition was up to this point basically looked like it was going be a carbon copy of Origins or DA2's gameplay, ok, yeah, I would be very worried.
But since everything I heard up to this point was has essentially informed me that I should be prepared to relearn a lot of things, especially with things like enemies not scaling being a thing, so it would be a bit silly of me to presume that this one change is the one that they have clearly put no thought into this despite it being on a very big change to how the game would play compared to previous titles.
Especially since part of the reason this could work or not is how enemy encounter design is handled. I mean, yeah it would be really aggravating to have to change spells/abilities every fight.
But since that would most likely require to be fighting a different major faction after every fight, I would hope that with the whole open world exploration system and enemies being logically placed and being possible to be warned of, Bioware would be smart enough to not mix faction fights outside of maybe a section in which you're meant to be running away and not actually engaging in fights anyway.