how to new and old iphone swap

I am going to buy a iphone5 for my son's B day, he has the iphone 4 now. He wanted me to use his NICE iphone 4 to replace my very old iphone 3GS. He is not in town.

So how can I do the swap and still keep all the info and number intacted? Can I buy him a gift card first for him to get a new phone. Then he send back the iphon4 for me to go to Apple to get my infor and numbers in iphone 3 to be add to iphone4. How do I back up the iphone? I usually do it in iTone sync, is it enough?

Thanks,

Li