you can't just put your sim in the new one because it takes the micro SIM. You're probably going to have to go into the store to have them change the numbers on the SIMs to match.
I'm still confused by all of this. So they'll change the sim of my newly added line to a number that is already on my family plan as long as I sign a new 2 year contract on the newly added line?
Also, assuming I'm a brand new at&t customer and I decide to open a family plan with 5 lines, I can choose the cheapest family rate and data packages for all the lines. Then I can sell 4 of the phones and cancel 4 data packages and I would be left with a family account of 5 lines w/ 2 year contract but only 1 active line w/ data package... is this correct? Anyone?