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.
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.