You hit champs that run multiple shields on themselves, like Shen with Courage, too hard that way. Or Galio.
And how does this affect things like Morgana's E or Sion's W, where the point of the shield is less the damage reduction, and more the secondary effect? And what if, say, Morgana's E is the bigger shield? Does it take that one, which doesn't block physical damage, or does it take the regular one, which isn't as large a number? And how does it affect Ivern's or Sion's shields, which do damage upon expiry?
Along that line, how does that affect Ardent? If Sona (who has Ardent) uses her W to shield the ADC, and then I ult said ADC as Shen (who had better not have Ardent), does Ardent cancel out? What if I ult first, meaning their smaller shield never comes online for Ardent to take effect? What about Athene's, does the shield still transfer that health?
What about things like Maw of Malmorticus, which you are literately incapable of controlling? You can't just go, "Nah, I won't use Maw's shield right now. I'll wait until after Janna's shield wears off".
How hard does this impact champions like Lux, who are meant to shield their entire team at once, when alongside a different champion that gives a larger, single-target shield?
And what if the stronger shield has a shorter duration than the smaller one? What happens then? Does the smaller one come online automatically after the larger one expires? Or is the smaller one just wasted? What happens if there's a smaller one that lasts for, say, 4 seconds, and then a 1 second larger one hits them? Does the smaller one resume immediately afterward, losing only the duration of the larger? Is it put in "stasis", so to speak (giving a total shielding time of 5 seconds)? Or does it just outright cancel it?
While this is the first relatively fair suggestion I've seen for changing enchanters on the boards, it has way too many questions that need to be answered.