The best way to port:
1. Get your iPhone activated on a new AT&T number. Keep your Verizon phone activated.
2. Try out the service for 30 days, or at least until you're certain AT&T and the iPhone will work out for you.
3. If you like it and all is good, call AT&T CS, preferably from a landline (1-800-331-0500) or from another wireless phone. Get a rep, and tell them you would like to port your Verizon number over to your EXISTING AT&T line.
The "new" AT&T number will go away, the Verizon account will cancel and shut off, and your Verizon number will get ported to your iPhone.
4. If you don't like the iPhone or AT&T service, just return the phone. Your Verizon service will obviously continue as if nothing happened. No stress in trying to get your number ported back.