Exactly...and that's the beauty of a SIM card: no real need to contact the carrier when switching phones, because it's the SIM that needs to be activated, not the phone.
AT&T's official policy is that GoPhones are not to be used to replace phones on contract accounts. If you plan to do so, do not purchase your GoPhone directly from AT&T. They will make you create a GoPhone account and add funds to the account. If you purchase the GoPhone from a local retailer such as Wal-Mart or other big box store, you activate at home, if you choose to do so.
Yes, although you will have to get a new sim. You cannot add a pre-paid account to a post paid but, a SIM is a SIM a phone a phone, as long as it is ATT. The gophones are loaded with feats that are'nt even meant for pre-paid, so, they are quite versatile.