Some provider will provide mobile porting code by just sending sms Port You ten digit mobile number. And in few minutes you will get code for porting. This code is valid for 7 days.

You just can't do that unless deactivate your account first.

i never noticed that... thanks

If you decided that you want to go with AT&T but you're not sure if you can port your number over, you should first check if the number is portable to AT&T. You can do this by checking here:
Eligibility Check - | Wireless from AT&T, formerly Cingular