...until you have had it a while longer if you haven't had it long enough.  You can only change it after a certain amount of time.  I don't know what the time frame is.

Hi xyz, a member can change an alias after 90 days (or every 90 days if they want.)
In the OP's case he should have no problem as he has used that alias for a lot longer than 90 days. Your info on how to go about changing it is correct.

...your current alias.  Then you type in the one you want.  You can try a previous alias and see what happens.