TL/DR Version:
Will I lose my unlimited data plan since I upgraded with another 2 year contract from AT&T to preorder the 6S+? I’ve never had a problem with upgrades before but this time got a message at the end of my order that said "All of your devices will be converted to your new Mobile Share Value plan within 48 hours after your pre-order device has shipped." I didn’t select and do not want a mobile share value plan, so now I’m worried. Did any other unlimited data plan customers have the same experience?
Happened to me last year. I panicked and called ATT and was told the same thing you were told. And it was true... My UDP was fine.