If you upgrade, i.e., sign a new two-year contract for a subsidized phone, on smartphones 1, 2, or 3, you will lose unlimited data. When the basic phone becomes eligible for upgrade, you should be able to upgrade it, and then move that phone to one of the other lines if you want, and then put the basic phone back on that line.
Be wary of reps that don't know what they are talking about. They say you can do it, you get the phone and next thing you know your unlimited data is gone. At that point, you cannot get it back.
You didn't say if your basic phone line was just a dummy line used to get upgrades or not.
If it is, you basically just use that line to buy a subsidized phone to keep one of your other lines on unlimited.