I would try this but I'm pretty sure they know I have no cards to play here - I just signed a new 2-year contract earlier this month on my new iPhone 6+. If I call retentions they know I'm not going anywhere due to ETFs. But I would love to upgrade from 2Gb to 10GB and actually receive 20GB for that price.
Never know until you try. The base price of $100 for the 20GB/unlimted text & talk has nothing to do if you are on contract or Next payments.