Based on what we've heard the requirement to become Red Mage will probably be beating Ultima Weapon at level 50, buying the expansion, then doing whatever the unlock quest ends up being. So yeah, if you want you should be able to switch over at that point. Youll want to get the appropriate caster gear, and you might have to stop for a short break to level up(i dont remember if they announced RDM starting at level 50 or not) to continue the story... but otherwise there shouldnt be any issues.