Like players have to undergo a quest once they hit 51 to become a new class, and they start over at level 1 (but the same age, same constitution) and have to reach the level of their skills but not re-practice ones they already knew?

Say, for instance, paladins, warriors, heavy-armor clerics and rangers could become some new class that blended the abilities of all of those classes.

It is a cool idea and I have seen it very successfully done before.