You have to be gone 6 months before you can become a new customer, AT&T or not.
Also to add to this is that you are still not considered a new customer. You will fall into the "Former Customer" category. Formers get some deals but not everything a new customer gets.
You will start seeing more and more of this "Former" popping up with all providers.
Why? To stop what is called promo hopping. It takes 2 years to break even on a new customer. If customers cancel every year, or even every 2 years, and go to the next guy the provider is not making money.
The providers created this monster years ago and are now feeling the effects. So their solution is to offer some sort of "good deal" for existing customers to keep them active.
To answer the OP's question I would do a Dishin It Up. You can get either a free or very low cost upgrade. If you're looking at a HDDVR, I would hold off till spring when the 922 comes out. This will be the best box on the market and will only run you $199.
Just my $.02