I think we are actually agreeing a bit more than you realize.
To state it a little more succinctly, it would indeed be a change in pace. A change that would change it into a game that may be good (personally, I always wanted to try a campaign in Dark Sun) but it would so broadly change the setting that it wouldn't be Ars Magica in the same way that Dark Sun wasn't D&D.
Further, and I'm willing to be wrong here, I think that the the amount of work to make the game setting make sense would be a great deal more than you envision. You said yourself that you are going for another advanced degree and, by implication of your chosen subject, are carrying a heavy workload. I'm not saying "don't do it" or "it wont be fun" . Indeed, I think that it would be a very interesting exercise for someone who has the time and, if done well, might be something I might like to play.
Consider the following cases:
Case 1: Disease
In the RAW, a fair number of diseases are caused by demons and one (at least) is caused by the Divine. If the Infernal becomes even just a little more powerful, would disease be more prevalent? If so, how? If not, why not? Now, if the covenant is 3 magi and a staff of 12 servants, maybe not a big deal until they had to go to town for whatever reason. Since towns, mechanically, are less healthy places, would they be even less healthy? If so, how much? If not, why not? And what about the lepers? Can they be healed? Are they healed? Are they all suddenly better? Do they all somehow forgive all the persecution or do they seek revenge. The easy way out here is to situate the centrl covenant somewhere far from any lepers in the first place, I suppose, but that starts limiting choices for the troupe.
Case 2: Old gods
Suddenly, the old gods (Baal, Apollo, et cetera ad nauseum -- herein assumed to be Fae in nature with a rare Magical god) CAN come back. Do they? Lots of persecuted pagans out there. How do they react? Are there "crusades" led by them and the suddenly more powerful entities? How do they relate to the Infernal? Do whole villages start sacrificing chickens, bulls, children, themselves to gain favor? If so, how? If not, why not? What does the common man do to protect themself from the supernatural and preternatural? Do they engage new powers for that protection or do they keep thier head down and hope for those powers to ignore all those (perhaps temporarily) unprotected commoners? Consider especially the motivation -- supported by game mechanics -- that more followers/worshipers/servants equals more power.
Case 3: Finding a level
If all the Divine auras drop at once -- and setting aside the issue of regiones -- how do the other auras change? Do they "pop up" in the spaces vacated by the Divine? Do, for instance, "the docks" where all sorts of petty crimes have been committed suddenly go from Divine 2 during the day and Infernal 2 at night go to Infernal 2 all day long? Or does it take time? And since all auras have some effect, mechanically, on the "personality" of an area and the inhabitants therein, how do they adjust to the sudden change? Or to the gradual one?
Also, if the aura power goes up for the other Realms, how much does it go up? And would the effect be immediate or gradual and why?
As the balance of power shifts, does it shift gently or violently?
Case 4: The Infernal
Since Hell was created by God, what happens to all the residents of Hell? Are they suddenly loosed? If so, then we need to resurrect the great Flambeau archimagus Bruci Campbellis -- talk about the walking dead! If not, then what happened? In a like vein, one might wonder about all the souls that went to Heaven. Can they be called back? Did they just disappear in the even that God disappeared in? May not be critically mportant in the start of the game, but could be a big deal after a bit as characters start dying.
And I'm sure we could both develop even more cases. Again, let me make clear that I don't think it is a bad idea in and of itself. I am, however, saying that the change -- from my perspective -- would be game-changing and require a great deal of thought and work. Even sagas that are done "in system" and with only scant attention to Medieval history require more work than originally envisioned. This would be a much grander thing.
But by all means, please don't let me stop you! Drop me a line when you get there, I just might be interested in playing a Guernicus who is Christian.