What smartphone can I use with my old, existing AT&T plan (blue plan) without being forced to change plans?

I have the old AT%26amp;T plan (blue) and would like to change phones. I've heard of people using some new smartphones without changing plans but not sure how to go about doing it. What phone(s) can I buy, how do I set them up, and do I only need to subscribe to mmode?What smartphone can I use with my old, existing AT%26amp;T plan (blue plan) without being forced to change plans?
old blue plans used two types of technologies: tdma and gsm. you can use any gsm phone with your old at%26amp;t plan if the phone you have now holds a sim card, but it has to be unlocked. All of the services that you have on your plan now, are no longer offered by at%26amp;t. You can keep the plan you have now, but if you change it, you will not be able to get it back, because at%26amp;t no longer offers it. Mmode is no longer offered by at%26amp;t. the only ppl who have mmode are the ones who've had it since the days it was available as an add on (more than 2 yrs ago)