Weird - I added a line to a family plan and it forced me to add a "tiered" data plan even though I have a "grandfathered" unlimited data plan on all of my lines - I went through and ordered it anyway and will call them and change when I activate the phone - Verizon has always been very good that way - I am not concerned.
The line that you just added will have to be a tiered data plan. Your other lines had the unlimited because they had it before Jan. Any new line have to go to a tiered data plan but if u had unlimited before the switch to tiered plans you get to keep it.