AT&T UVERSE/DSL question about promotions and new/old service swapping?

I was wondering if someone could purchase u-verse at a promotional price for a residence that already has existing dsl in another person's name, then once they receive the self-installation kit, cancel the DSL and have them activate the uverse? The first person is kind enough to allow the DSL to stay on until the new internet is installed so there is no lapse (or at least not a big lapse) in internet. But would ATT allow it or does the DSL have to be shut off before the UVERSE purchase can be started and if so for how long to switch account holders? I attempted to ask their customer care online but the person was obviously confused, they thought I was asking if one was able to add a different service (like a phone or tv) to an existing account. Has anyone done this before?