I know many don't like to hear this but I could imagine that they just change the MP costs on their spells. AST spells always costed less MP than the SCH/WHM equivalent but it was justified as the potency was lower. Now the potency is the same and the spells still cost less MP.
I'm actually okay with Astrologian right now, healing wise. I seriously wouldn't mind a better card system, a reliable one(the retroactive spear idea as an example would be great). And a small duration buff to disable would be nice.
I'd also like it more if they just change the MP costs of SCH and WHM spells to the cost of AST spells lol.