i've added lines while in the midst of my 'unlimited' contract. the new line WILL be subject to its own 'limitations', in terms of data cost/usage. adding the new line will get the additional $10 fee, plus whatever type of data plan you want for it....but the original line with its grandfathered unlimited data is NOT affected. only the 'new' line will be.
a grandfathered line will 'supposedly' never be revoked UNLESS you buy a SUBSIDIZED phone directly from verizon, because the supsidation is tied to brand NEW two year contract, which effectively wipes out your current 'unlimited' plan, and thus is subject to whatever data package is being offered at the time.
as long as you do NOT purchase a subsidized (discounted) phone directly from verizon, you can swap phones any/every day WITHOUT it effecting your current, ongoing unlimited plan on your phone number. adding a 'new' line has nothing to do with your 'current' line.