I activated a flip phone for my son, good old LG VX8300, all he needs is voice and texts...
My wife and I and my son now share a plan, both my wife and I are unlimited data (she has an iPhone4S) and I have a PrePlus, when I activated he joined our shared plan for voice and unlimited texts.
However, he does not have unlimited data ($1.99 per MB, what a rip off), so I blocked all data to his cell.
Now, when I talked to Verizon, they told me I can keep my Grandfathered plan (which will turn month-to-month) at the end of the month, as long as I can until I switch to a subsidized phone, if I use a phone on ebay/amazon/some store (just buy the phone and not connect it to Verizon), then use the switch my device, my existing plan stays...
Now if I miss a payment, or cancel my contract (even though it would be month-to-month), then I will loose the plan.
So here is the kicker, a new phone with a 2 year contact free, but the shared plan as I can use up to 12G in one month (not including tethering 5G free with hotspot), we pay a premium for keeping unlimited 3G (as the PrePlus can only use 3G).
FYI I have heard others say if you go LTE you have to switch to a new plan, but I do not know this to be true and the Verizon Rep I talked to said that it was not true, but YMMV...