THis is how it works.

Say you have 4 lines.

2 lines have unlimited data(A and B) and 2 lines don't(C and D)

You transfers line A over to line C to upgrade

You don't lose unlimited because of a software loophole.  If Verizon audits they can remove unlimited without reprecussions.  Line A has to agree to the new terms and it is known accepting the new terms unlimited is not an option.  Only Line A accepts the contract and terms and conditions.

So the only option is to upgrade line C and then move the phone over is the safest way to do it without losing unlimited.