Carrier ATT next upgrade and multiple order question

Ok so here is the situation, I bought the 6S through the ATT next program 20 month program. How does the upgrade work and where would I have to preorder it from?

1. If I can upgrade my 6S, what happens to the 6S, do I send it back, or is it better to pay it off and then get back into a new ATT next program? or should I get the Apple upgrade program.

2. One other line in our family plan is also eligible for upgrade, so do I get that also in another ATT next program or apple upgrade program?

3. On the home page I read that the Verizon and sprint version are going to be true world phones?
I also need to buy 2 phone for my family who are going to be visiting USA, how do I pre order for them, I went through the pre order screen and I had to select the network first, how does the unlock version work? is the unlocked version available for pre-order?