my mom is getting an upgrade to the iphone4 and might give me her iphone 3GS. i already have the Xenon. i like it alot but the iphone is soo cool! i was wondering if i could go back and forth between the two. can i take the sim chip, out of one and put it on the other and see which one i like more instead of chunking the xenon and finding out i dont really want it??