PAC code and cancelling contract in correct order. Help!

Bit confused as to *when* to cancel contract on 3 and request PAC code.
3 CS said I must phone on 13th Nov to cancel the contract (which ends
the next month in Dec).
Now if I want to keep the number and port out to a PAYG sim on another
network (e.g. O2), what is the correct order of doing this and on what
date so as to minimise paying for extra days not needed.
Can any PAC "guru" please assist?
Thanks very much.