You can use an available upgrade on your Dad's phone to allow you to upgrade your phone. Just go to the store with him (assuming he is the account holder) and let them know up front. Keep in mind that the iPhone 4 uses a micro SIM card which is incompatible w/ older SIM card slorts (unless you make or purchase an adapter for it to work in an older phone). Technologically, the micro SIM is no different, it is just smaller than a regular SIM.
There are two cell phones on my plan. I have the iPhone 3G, and my dad has some Razr phone. I would be eligible to upgrade to the new iPhone4 right now, but my iPhone was stolen last year, which means I had to reorder a new one for $300 last year. So I won't be eligible for a full iPhone upgrade until early 2012. My dad is ready for a full upgrade next week. Could he order the new iPhone 4, and then we switch sim cards? I would take the SIM card out of my iPhone, and put it in the new iPhone4, and he would take his sim card and put it back into his original phone, or take my iPhone 3G.
Would this work?
Thank you everyone for your help!