Huh??? DOCSIS requires backwards compatibility, both for the head end and the modems themselves. Any DOCSIS n hardware is compatible with n+k and n-k for all values of k. There's absolutely no reason for your cable company's head end to not negotiate a connection with your existing cable modem. You just won't get the faster speeds provided by the newer standard.
Maybe that's true, the same thing happened to me: alleged backwards compatibility didn't stop Comcast from causing my DOCSIS 2.0 (Linksys BEFC-MU10) modem to stop being able to connect. The DOCSIS 3.0 replacement (Zoom 6341j) I bought worked.
I wouldn't be at all surprised if Comcast intentionally configured their network to reject connections from DOCSIS 2.0 modems even if they were supposed to still work, in hopes that some people who owned modems would start renting (or just to punish people for having the audacity not to rent). It's just the kind of thing those criminal, corrupt fuckers would do.