Thats not correct. There is an entire thread dedicated to transferring upgrades and keeping UDP. I've done it as have many.
Start reading here.
[Instruction] How to Keep Unlimited Data on Verizon - Android Forums at AndroidCentral.com
Yes, you can transfer an upgrade from another line to upgrade an unlimited data line. But if you choose to transfer the upgrade that's for an UDP line to another line, that UDP will need to be changed to a current plan.
The thread you are referring to is telling people that have a UDP line to either add or use ANOTHER line's upgrade that you don't need UDP on, to get a phone for the UDP line.
Line A: UDP
Line B: Basic Phone with upgrade
Use Line B's upgrade to get a new phone for Line A.
Unless I'm horribly mistaken, you
cannot do:
Line A: UDP Line with upgrade
Line B: UDP Line with upgrade
Line C: xxx Line
Line D: xxx Line
Line A transfer upgrade to Line C for a new phone and keep UDP.
The UDP will be removed if you try to upgrade the UDP line (unless of course you go through BBYMobile or there's another loophole/glitch on Verizon's end). Again, unless they changed it since... April, if you transfer an upgrade, the original line that the upgrade was for will be renewed at 2 Years.
Please correct me if I'm wrong, though. I just assumed that thread was for utilizing a Basic Phone line to upgrade an UDP line.