January this year a line on the account (2GB/mo data plan) was due for an upgrade. I had them upgrade to a Moto X (via Best Buy) for $1 (+$30 upgrade fee). Once activated, I swapped phones. Purchased Developer Edition at full price, sold the $1 Moto X, and ended up with a Developer Edition Moto X for like $100 and kept my unlimited data plan while the other family member got an upgrade (my old Galaxy Nexus) from their Droid X.
Today... because another family member can't take care of their phone and the other family member used a crappy charger with the Galaxy Nexus causing the phone to flip out (touch screen problems?), I decided I'd give up my unlimited for the 6GB/mo plan. That'd let the one get the Galaxy Nexus which is better than their broken device, the other to get an upgrade (repayment for letting me steal their upgrade), and I'd be back to being able to get new devices with the subsidies if I wanted to.
Called a local Verizon store and they guaranteed me that by transferring my upgrade to the other line I'd lose the unlimited and get the 6GB/mo plan. Called the Verizon mother-ship and again was told I'd lose the unlimited data plan.
Also, for fun, the data usage analysis in the My Verizon account is more accurate that what the Verizon sales reps see. Last month I used 6.22GB according to my account page. Sales rep said it was 8.4GB. His reasoning that the account analysis is more accurate is that other lines on account that have 2GB/mo were showing 2GB used whereas his screen was showing 3GB yet we weren't getting charged the overages. Not that this means much, but... perhaps when Verizon says they're going to start throttling the top users, assuming they're using their numbers, this could mean they're saying you use more than you actually do, therefore, you might be getting throttled when in reality you may be below that cutoff.
Anyway. Knowing my fate, I let it happen. 2 hours later my family member calls me saying somehow I still have unlimited data. I called the rep and asked and he said he didn't know how it happened. Guy said I got lucky or something. He also brought up the closed loop-hole of upgrading a dumb phone line, swapping devices, and putting a dumb phone back on. He said that you should still be able to upgrade a dumb phone to smart, swap devices, and keep your unlimited plan.
Long story short, it seems like if you're buying devices at full cost to keep unlimited data and you're sitting on an update another family member could use, you apparently won't lose your unlimited data by "renewing" your contract. At least that's what happened to me today.