I have never run into any problems with the reps at the Verizon store not allowing me to purchase a phone at full retail and keep unlimited.
I buy all my phones at Verizon store full retail in order to keep my unlimited data..never once have I had a problem keeping my unlimited plan intact. What I have noticed at best buy full retail prices are anywhere from 50 to 100 dollars more than Verizon store prices off contract...go figure.