Replacing the Magic System Entirely

[MPHopcroft]

If I wanted to do an alternative d20 template that entirely replaced the way spells were obtained and used, for a world-specific campaign, how difficult would that be with the tools available now?

And will there be tools that would make that job easier?

An example would be a system that perhaps uses the same spells, but instead of awarding them by level gives mages a pool of "magic points" from which to buy their spells.