How easy is it to switch a different line in a family plan to my old 3GS?

[JoshGlzBrk]

So my iPhone 4 should be here tomorrow. (hopefully) He spent a day in Alaska humping Sarah Palin I think but he is in Oakland now... just needs to get to Seattle

Anyways I know my number will be switched over to the new phone when I activate it but how easy is it to get my family member's line working on the iPhone 3GS? She has a 3G phone but no data so will I be able to just put her sim in the iPhone and add a data plan on the AT&T website?

 

[Geckotek]

Yes

 

[iMrNiceGuy0023]

I was wondering the same thing too, someone wants my old phone, since there are 2 data plans will AT&T automatically just add the $15 plan once the sim is inserted ?