You can go in and demand the "new customer" deals. I've done it several times with Comcast. It's a pain, but they will do it. Best to do it in person not on the phone. It's not possible online AFAIK.
As these people are in the minority, the companies can afford giving them the goodies - the "suckers" foot the bill regardless.
Sadly, everyone's favorite darling Sonic.net doesn't do this. They will gladly charge long-time customers more than the advertised rate though.