Are you pre-ordering via Apple.com or did you plan to use your new carrier?
If you are sure that your new carrier choice has good coverage where you plan to use your iPhone then when you pre-order, you just enter your AT&T phone number and the other account information that is required on the form and your current AT&T phone number will be ported to your new carrier.
You can always order a new line for your new carrier, you will get a new number and then you can try out the coverage and then do a port of your AT&T phone number. This would be my first choice and it is the lowest risk.
Just a note, once the port completes your AT&T account will be closed so make sure that you understand any ETF fees before you go this route.
Dave