[Sawrock]

Once I get a character grounded in a set of two classes, I usually keep them in those classes until:

A: A new class that comes out that is more fitting for the character.
B: I think a class I originally chose for a character doesn't fit a character too well.

Now, this is just my personal opinion, but even having a third class one goes to is too much. It's like having a doctor-mechanic-farmer, it's not possible to have so much training in one's life. And as for the characters who are extremely old, I'd note that it'd be likely one would forget previous fighting styles after dedicating themselves to new ones in intense enough training required for a promoted class.

As for the general problem of omniclassing, I'd personally prefer if we had a mini-destiny system, where we could lock two promoted classes to a character (and their respective base classes, as well). It wouldn't be as useful as destiny classing (so not to invalidate it), but something to help against omniclassing, mechanically, would be good in my book.