Originally Posted by yulscs

WWCP bookings can be changed, but not online. You have to call the reservations call centre. I did this a couple weeks ago to change the dates for a WWCP booking (the pass was already expired, like yours), and it was no problem. I haven't changed the origin so I can't comment on that, but both are in Europe (same WWCP zone) so I would think it should be okay.

Be mindful of the validity period of the ticket, though. Is September within the 1 year validity from the date you created the booking?