asked my CS rep that exact question seeing as I am in the same boat with 2 smart and 2 basic, one unlimited and one 2/4gb current. Answer I got from this particular
cs was if you buy it off contract you can use the 700 or 1400 plan with data pay as you go or upgrade to share for all for everyone. In other words yes your grandfathered but change anything plan wise and we got you.
If it's a dumbphone, it wouldn't matter. You can still buy on contract and not be forced to change your whole plan. The only thing that IS changing for existing customers is that unlimited data won't carry over if you currently have it and upgrade. You don't necessarily have to switch to a Share Everything if you upgrade but you'll need to replace your unlimited data package with one of the existing individual line tiered data packages. At least for the time being anyways. Who knows what Verizon will decide a year or two down the road.