Question about transfering numbers to an active account

Ok so im currently with at&t and i plan on moving to verizon when my billing cycle ends which would be the 24th of feb. I wanted to go ahead and order the device and plan tonight that i have in my checkout. But verizon is asking me if i want to port my existing number or get a new number. I'd like the same number but i want to finish up my month with at&t. If i choose port number option on the site will they port the number right away or wait til i get the phone in my hand and then do it? or if i select get a new number, once i activate and enable the verizon account can i transfer my at&t number to the account after the fact?