Right. Although my plan is from the former AT&T, even before Cingular bought the AT&T wireless division.
As far as whether I could duplicate it with another carrier, I don't think that I can, though I haven't really looked into it very deep. For the most part I'm fairly satisfied with AT&T.
The only reason I asked was whether it was one of those plans that was too good, and that's why you are worried ATT will take it away. You could always call customer service and inquire as to whether you would be required to choose a different plan. My guess though is that you are not set up in the same way as someone who has already bought a new phone from AT&Tingular, as those people (even in grandfathered plans) probably already have newer SIMs and such; the system might not let you re-register your old plan on a new SIM.
I know ATT "Blue" had a lot of integration issues and my gut is that you're probably right on about being stuck for an upgrade. If your plan is such a great deal that you can't get it replicated on (imho) ATT's current excellent plans, then they are probably just looking for a reason to trap you.
What are the details of your plan that it's so great? Do you get all the current ATT benefits like mobile to mobile? If minutes are your issue, have you thought about going with a higher plan than you need and banking the rollover? Just curious what keeps you from changing.