Although you have a point about it teaching a lesson i have to say that the idea proposed would be a lot more fair and newbie friendly. People who want to change class are either newbies who learned something that makes them want to change it/didnt like the new class or ppl who made a mistake while multiclassing...which happens...ive done it too
I might even go so far as to say that you should be able to abort after tri-av, back to 50/50. Ive had it a couple of times where ive remade a character in order to replace one class. Its a bit of a drag on crafts and such
(fyi, usually because of patch changes etc..not because i didnt know what i was getting before i picked it...)