Swapping sim cards

[penance]

I have an unlocked first gen iphone and I'm interested in purchasing a new 3G iphone. Here's my problem ... my current account information is that I'm on the unlimited media for internet & texts, not an iphone data plan so it doesn't recognize that I'm using an iphone.

My dad is telling me to go to an AT&T store, tell them that I lost my iphone (his iphone actually when I give them the account info) I buy a new phone, they pop in a new activated sim (with his number)

Now that I have a new 3G activated iphone but with my dad's sim, would I be able to take out the current sim and replace it with mine? and have it work fully functional while still using my current AT&T data plan?

Thank You

[penance]

bump ... I hope I can get an answer I have to go to an AT&T store today to get a new iphone asap as my old one got vomit on *cries*