Eligible ATT iPhone Upgrade SMS

I have been an AT&T customer for several years. I bought my kid an iphone several years ago with an unlimited data plan. After a couple of years the iphone broke and my daughter wanted to try another phone. She did not like the new phone so I told her we would get another iphone. Because we were later than one month by just a few days after the new phone purchase, I could get another iphone by paying full price, but would no longer be aloud the unlimited data plan I had maintaind for years. I have a 3 party family plan and intend adding a 4th line, unless AT&T change thier minds because the way I was treated by AT&T (not grandfathering in the unlimited data on a new iphone), I will be purchasing a sprint EVO with unlimited data for my 4th line.