If you are used to amending your vehicles online but are unable to on this occasion, it could be that when you booked your parking you inadvertently selected the 'Parking' option instead of the 'Permits' option.
All standard Westminster City Council parking sessions, up to 31 days and purchased by phone, mobile apps (not local authority based permit sessions), text or online, are non-transferable between vehicles, as per the parking operator's rules and regulations. Therefore, if you have purchased a standard session and require the active vehicle to be amended, unfortunately you will be required to book a new session for the date(s) which you intend to use the alternate vehicle and seek a refund for duplicated date(s) directly from the parking operator.
However, if you have purchased a Westminster City Council Permit, it is possible to amend the active vehicle(s) on the session at any time up until the permit session expires. The system has been designed to accommodate vehicle amendments on permits (longer-term sessions in excess of 1 month) only, as requested by the parking operators to whom we offer our service.