Edit: This only applies to accounts with unlimited data. If you currently are on a share everything plan, a dummy line will have to be $30
I recently looked into this. This is indeed possible.
First of all, if you currently have unlimited data and on a family plan, you should NOT have the Family Share Everything plan. This is a new plan, and your line cannot/should not be on this plan, because that means if you have more than one phone, you have 2 SEPARATE plans, (1 for unlimited data line, and another plan for your other X phones). The plan that you should be on for a family plan with unlimited data line is the Nationwide talk and text (unlimited text and 700/1400 mins). This plan you share minutes/texts, but data is individual to the line. On share everything its 30 dollars access charge for a dumb phone, BUT on nationwide talk&text, its 10 dollars just to have the dumb phone with no data on it.
I spoke a knowledgeable verizon rep that did a bunch of calculations for me asking about these type of scenarios. Doing this costs a few hundred less than purchasing the new iphone at full retail. because its $240 + phone cost + tax vs. $649 + tax.
What you have to do is add a dummy line and start a new contract on that line to get a subsidized device. Activate the new iphone on the dummy line first. Let a few minutes go by, and walk into a store/call CS to ask to do a device swap. Then you will put your new iphone on your unlimited data line, THEN what you need is to find a dumb phone that is not a smart phone and give the IMEI to verizon in order to give them a phone to tie the line to. If you currently have a smart phone and you swap it in to the dummy line, they will detect it and add a data package to that line which is like an additional 20/30 bucks. Hope this helps, I am going to do this once I am able.