Just bought the LG G2 last night, for $49.99 + $12 tax. Costco only charges tax on the wholesale price, whereas through Best Buy or Verizon the sales tax is charged based on the retail price... which would have been closer to $50 instead of $12. There's also the $30 upgrade fee which will show up on the bill.
Anyways, my UDP should still be intact, and we used my line's upgrade. But I have a family plan with several dumb phone lines (no upgrade eligible) so we took advantage of that.
Where's more-or-less what they do, and this is actually a standard practice even through Verizon (if you come in contact with the right rep):
1) They transfer the upgrade from my UDP line to a dumb phone line.
2) They activate the new phone through their system with a dummy SIM (not my old one) on the dumb phone line.
3) They re-activate the dumb phone on the dumb phone line (they don't actually need the dumb phone to do this, but that line will be unavailable to the dumb phone for about 5mins).
4) They used my old SIM card to activate the new phone on my UDP line.
So in the end the net result is like the result of the steps in this Best Buy thread: I use my upgrade to get a new phone on my UDP line, get a 2-year contract extension, and keep UDP. So far so good.