I never assume that there's only one way to cast spells in a setting. I know it would be more convenient if I did, and allowing multiple methods of magic might conceivably unbalance or 'mess up' the campaign in some way, but in my experience it's never done that. The main thing it's done is help remove the mundane from the magical.
I firmly believe that magic in any work of fiction, including an rpg, will always suffer if it's well known and understood. Quantification is the bane of imagination. There will always be those who clamor for all the mysteries to be revealed, however. For those, I offer... midichlorians.
I think Lucas was acutely aware of what he was doing. In Star Wars IV, Luke finally went to Ben and said he was ready to start learning. The next thing we see is some unspecified time after that. A certain segment of the viewers cried out in dismay. "I want to know what they did!" "I want to see what he taught him!" "I want to be a Jedi and learn the ways of the Force!" Well.. you can't learn the ways of the Force. It doesn't really work. It's made up. And the only thing you would have gotten if we had seen what Ben showed Luke is concrete confirmation of that extremely immersion-wrecking, mystique-busting fact. The clamoring never stopped, only getting louder and more pervasive. Then, I really believe Lucas set his jaw and said "fine! You want concrete? Then let me prove my point to you! Kazaaang! Midichlorians!" And I think the quantifiers got exactly what they deserved.
So, here we have the extremely well quantified and detailed magic system of DnD, particularly in 1st edition Advanced D&D and afterward. It's quantified for a reason, since it's part of a rules system. However, contrary to the proclivities of some, this concretism isn't self-justifying. Introducing the idea to players that more than just the one magic system they already know may exist and work pulls the rug out from under familiarity and stagnation, while at the same time leaving the foundation of reliability inherent to their current, well-delineated system. It kills midichlorians!
Players in my campaigns still start with whatever basic, default magic-use system is present in the setting. After some experience, they gradually learn there are other functioning systems out there. They may have started with Vancian or an elementary spell point system, but then the wider world of possibility opens. True-name magic ala Earthsea. Lovecraftian tomes that any can attempt to use, but at a terrible price. Spellsingers. Deal-making spirit binders. Leyline weaving. Artifact channeling. Rune-casting. The only real limits are the bounds of your literacy.
The trick then, of course, is to introduce it in a subtle way. The PC's actual first exposure might be dramatic indeed, but it needs to be handled in a way which won't lead to watering it down through proliferation within the campaign. For it to mean anything, it has to stay incredibly uncommon, weird, even alien. A wizard in the party may indeed be able to learn some of the fundamentals, even mastering a spell or two. However, there are two keys to this. One: it's going to take a long time, or exact a heavy toll. It took them years to finally become a lowly first level wizard in the first place, and this certainly isn't going to come to them overnight. If time isn't an appropriate commodity, then something else will have to give. Anyone can pick up the Book of Eibon and try their best to copy the formulae, but they're certainly not going to pass through that experience unscathed. Gold doesn't count as a heavy toll, either. This is the realm of insanities and permanently lost ability score points. Two: there usually needs to be a willing teacher. The wizards on Roke Island aren't going to trust just anyone, either.
So far in my own campaigns, the far more common occurrence isn't actually learning the new system of magic itself. Rather, they have picked up a few of the side-tricks these other, strange dweomercrafters use. This includes things like how to enchant valuable gems as spell storage receptacles (an idea from Mythus.) The main upshot of all these considerations is simply this. Don't wreck your campaign, but take away a bit of the certainty and banality from something that's supposed to be 'magical.'