One thing to add to this is that it's very very inconsistent between different regions. AT&T does not seem to have a standardized process and how they will handle this situation varies wildly depending on the local office in question.

Best foolproof bet is usually to order a 2nd DSL account then do the DSL disconnect on first one once it's setup and then initiate the port.

Sometimes they are more cooperative. In some AT&T regions, they will still process the port and automatically/seamlessly convert it to dry loop.