Silly Idea for Demi-humans Changing Class

I don't like the possibility in 3rd edition of any PC acquiring new levels on the spot. In my mind, if a human is born a Warrior, he will die Warrior. That's it. Demi-humans though are quite a different story. Dwarves, Elves and Halflings have very long life spans. It makes perfect sense in my head that a Dwarf, having reached the highest possible level as a Fighter, decides to re-invent himself as a Thief instead of retiring. So, this is my idea (all rules refer to first edition, but you can adapt the concept very easily).

Demi-humans Changing Class: Once a demi-human character (except Half-breeds) reaches the highest level in his/her Class, he/she can decide to pick another Class and continue adventuring instead of retiring. He/she will add the new hit dice to his/her previous hit points, use the best To Hit value, use the best of each Saving Throw, use all his/her previous abilities along with the new ones, with the obvious limitations (a Dwarf Fighter 7/ Thief 1 will not use his new thief abilities if wearing Plate Mail). Will also acquire new weapon proficiencies, according to the new Class. XPs are as if the character continues advancing in his/her previous Class, but the bonus is now the one from the new Class.

Example: Górin Trollterror, a Dwarf with 16 Strength and 13 Dexterity, reaches the 7th level as a Fighter. He is still young enough to continue adventuring, and so chooses to pick up new levels in the Thief Class. Once he gains 125,001 XP (Superhero), he becomes a Champion/Rogue. He adds 1d6 hit points to his total (adding his +1 bonus to hit dice), and gains the new awesome abilities. No changes to his To Hit and Saving Throws values. He also gets 2 Weapon Proficiencies, but still has the -2 penalty he had as a Fighter with weapons he is not proficient with. From then on, Górin will no longer have the 10% bonus to XPs, for his new Prime Requisite is barely 13. When Górin has gained 250,001 XP (Lord), he becomes a Champion/Footpad, and so on.