I have talked to and heard from many others to be able to say this. It all depends on how lucky you are whenever you call. When I wanted to port over to Verizon from T-Mobile it took me 4 different reps before I found out that this is possible. Thats when you change carriers, but want to keep the same phone#. It takes either somebody who knows what you want, or can transfer you to someone who knows how to fix your problem. Some reps just flat don't care at all. Some have enough experience and the desire to help.
Given the current overwhelming sales of DX, I think some just throw their hands up in the air.
