I think class change tiers the we have now are fine, no need to change that.
The issue is with people who want a class tier that is above his current class tier.
My idea is: allow people to go back only one tier back at a time and with following conditions:
1) current char needs to be lvl 80
2) lvl afer change: 74 100%
3) no materials invested returned (books, etc)
4) maybe make it limited event (2-3 months?).
this is just an idea, conditions can vary, but it shouldn't be free to change class to a tier above current one.
Personally I think you should start a char from 0, but I understand it is for the good of the server.