I'm not wrong on what I'm explaining, it looks like we're actually pretty much talking about the same thing. From what you've described in your first post, you are doing exactly what I'm talking about to keep unlimited, which is swapping SIMs.
What I was trying explain for you or anyone who could read this in the future is, don't make the mistake of using online transfer tool. If you do it that way, the line receiving the upgrade will lose the unlimited data (I misread your post and assumed your other line had the upgrade not yours). Even in the sim swap scenario, if your other line has unlimited data now, it will lose it in this process. There is no way to purchase a subsidized device on Verizon and keep unlimited on the line that actually used the upgrade (unless you can somehow convince a manager or rep, which is a long shot at best). So as long as someone in this process is OK with not having unlimited afterwards, you're good.
When I did my upgrade, my mother-in-law's line had an upgrade so I bought the phone on her line, and did the sim swap. In her case she didn't have a data plan at all, so there was nothing to lose. But when my wife wanted a new phone, I used the online tool to give her line my upgrade. In that case she lost her unlimited data since her line got a subsidized device.
The sim swap thing does have some issues though. After I got my S4, I had an issue with the wifi. I brought it to a Verizon store and they had to swap it out, and it was a massive pain because the phone wasn't being used on the line it was purchased on. They seemed completely puzzled by this fact. To give me a new phone since I was in the 14-day window, they had to refund me my money for the original purchase, then re-charge me for the new device (and in the process I somehow got 7 cents difference back due to some kind of tax difference between my local store and online purchase), activate the new one on my mother-in-law's line (who then lost service for like 20 minutes in the middle of the day without me informing her....oops), then do the SIM swap again. Took me an hour and half to get that all resolved.