Upgrade swap scenario

Hi all, I’ve read the following link on upgrading swapping, but I have slightly different circumstances:

 

http://www.wireless.att.com/learn/articles-resources/iphone4upgradesupport.jsp

 

Me and my mom are part of a 4 person family plan. I’m not eligible for an upgrade, but she is. I was hoping to use her upgrade to get the iPhone 4 at Radio Shack this week because it’s $50 off. Now as I understand it, Radio Shack activates the phone during the purchase, so the SIM will get locked to her number.

 

What are my options in this case? Can I take the phone to an AT&T store and get a replacement microsim with my number?Another thing to mention is that I live away from my mom, but both of us are authorized to make changes on the family plan.

 

As soon as the iPhone is activated, I assume she will lose service on her old RAZR? What’s the step for getting that phone working again? Will the sim reactivate once I get a new microsim, or will she have to get a new sim as well?

 

Also, are replacement sims free? I’ve heard differing answers, some say you can just go in and ask for a replacement sim for any reason and they’ll give it to you, while I’ve heard others report charges ranging from $15 to $30.