i wil try and explain this in laymans terms, however vodka rules at this tim eof night
1) Your due date is 2nd of the month - every month
2) You ask for your migration date - anytime of the month
3) You migrate on 4th of the month
4) howvere afaik you need to give 30 days
therefore your payment is still due on 2nd of the month, granted you only use it for 2 days, you then have to request a refund for the remaining 29 days.
Do this by phone on 0845 1400200 and also raise a ticket to confirm.