I ordered my M8 through the BB website with the "keep my existing plan" option in late March. The new contract end date appeared within a day on the VZW website. I received the phone in the mail and activated it on my cut-down Rezound sim on April 2. It's now 3 weeks later and my VZW account still shows the new contract end date, my M8 as my device, and unlimited data. I won't be completely convinced until I receive a bill with an entire billing cycle (16th - 15th for me) on this new contract but so far so good. There's also a reliable source (an XDA moderator) who used this method to keep his UDP while purchasing a One Max a few weeks earlier than I bought my M8. He hasn't reported anything negative either.
My grandfathered UDP happens to be one of two lines on a family plan (the other is grandfathered as well) but I don't think that has anything to do with my apparent success.
I chose to buy a Noosy cutter to be safe (scissors just seemed crude considering my perceived risk of losing my UDP should anything go wrong) and it worked like a charm. NFC didn't work but I just took my new phone into my local Verizon store and told them I bought the phone elsewhere and used my old SIM just to make sure it was functioning. They got out a new nano sim and activated it to my current account and I was good to go. Even that was over two weeks ago with no changes to my account info as of a few minutes ago.
If anything changes, I'll be sure to post it here immediately as well as the Rezound threads. I have quite a bit of lingering loyalty to that great community. :highfive:
I had purchased a cutter to trim my sim down to micro (when I snagged a M7 on the April 1 promotion), but then decided it really wasn't worth the money to buy another when I returned the M7 for the M8. If you're careful, shears work great (but, I'm a machinist/fabricator by trade, and very meticulous about details and dimensions). There's nothing crude looking about my cut-down sim.
I'll be very interested to hear if you guys are able to fly "under the radar" on your upgrades, and not lose your unlimited. I got my M8 by using my wife's upgrade (she's still rocking a Rezound), but may consider getting her a M8 if/when the red Verizon version becomes available. Then, I'll have to transfer/use my upgrade to buy her phone.