Renewing my SERO contract

[kernel610]

I have been a sprint customer for about 7 years now and 2 of those years on SERO. I currently have a Touch Pro and I'm looking to move to an Android phone in the future. From what I've seen on the forum, it looks like some people are able to stay on their $30 SERO plan with cheaper more basic phones, but not with smartphones. Also it seems like its certain smartphones. For example, I was offered an upgrade to a TP2 and still keep my SERO plan, but I couldn't get a Samsung Moment and keep the SERO plan. What is it that determines what phones are SERO eligible and which ones are not. Are there any tips you guys recommend to get a Moment or similar phone on a SERO plan or any incentive I can get out of them for switching from my SERO plan? Thanks!