I've had a few different ideas on how to accomplish this, many suggested by commenters on the previous post:
I could simply create a cleric variant that is closer to the magic-user in terms of its hand-to-hand combat effectiveness - weapons, hit points, and the like - but retains the same spells. The "flavor text" of the class would be changed from an armored servant of a god to a magician specializing in spells of protection and healing.
Another possible course of action goes like this: Eliminate the spellcasting classes as they currently stand. Lump all of the spells in the rulebook together and then reassign them to new spell lists, each with its own class. My guess is that there would be at least two, possibly three or four classes when all was said and done - maybe a "white mage", a "black mage", and a "witch" with illusion and nature powers? This solution is one I've considered before, but it's not without its own potential pitfalls. For one thing, I would have to figure out exactly what kind of spells the ranger and paladin classes receive at high levels. Also, I'm worried it might be overly complex. Something about the idea smacks of 3rd edition D&D to me. That's not the feel I'm going for.
More or less independent of these possible courses of action, I'm also thinking about creating a "universal" spell list - a dozen or so 1st-level spells that any magic-using class would be capable of adding to their spellbook and casting. I'd like to make it so that even an attack-oriented class like the "black mage" would be able to cast a simple healing spell, and that a "white mage" could likewise zap a foe if need be.
An even more radical idea would be to split up all of the spells into themed lists, like "fire spells", "detection spells" or "healing spells" and give spellcasters a choice of one or two lists (possibly with access to more as they increased in level). This is similar to the approach taken in Rolemaster. It's also reminiscent of the cleric's "spell domains" in D&D 3rd edition. This idea appeals to me on a mechanical level, because it would make D&D magic work a lot more like some systems that I've enjoyed in the past, but again, it'd add a lot of complexity and might be wrong for the old-school feel I'd like to evoke. It would also mean a lot of tinkering with the classes, and frankly, I don't know if I'm up for that. It'd take time and expertise that I'm not sure I have at my disposal. (The irony that it would mean I had basically turned the magic-user into a cleric, rather than the other way around as I had originally proposed, is not lost on me, either.)
I'm a little worried that these are all potentially game-breaking ideas, and I have to admit that I'm starting to wonder if I wouldn't be better off just leaving the classes as is...