I also have an older 3.0 modem that I was using on Cox's 250 plan (for a long time). A few months ago I came across their new certified cable modems document to find out that my modem was now only supported up to 150. Unfortunately Cox seemed OK letting me continue to pay for 250 - I've downgraded to 150 for the time being, but only because I noticed the issue, not because Cox came to me and told me of the change.
Yes, my modem is old and I'll eventually get an upgrade to 3.1, but I'm a little disappointed that Cox wasn't a little more proactive once they changed their standards.