Carrier AT&T: How to upgrade another line on a family plan without interrupting its service?

[Nikos]

I have three lines on an AT&T FamilyTalk plan that I use to rotate upgrades every year. This year, I used an upgrade for a line that is being used on a basic/dumb phone.

I forget exactly what I need to do to use the new phone without interrupting service to that line. Do I just pop out the SIM and insert my iPhone 5S' SIM before turning it on, then call to cancel the data plan for that line?

I believe this is what I've done in previous years but don't recall as I used my own line to upgrade last year, and the year before that involved getting a Nano SIM, as the 4S had a Micro SIM.

Thanks!