Just chatted to Apple support and, strangely, she said that they are doing things differently this year and, if you opted into monthly installments, you have already completed the order and that's why your pre-order shows an expired; in other words, nothing to be done now and should be getting an order number (an actual phone) within 24h, before pre-orders even go online
bit of a strange one, I've been told I don't need to go now and complete pre order tomorrow, because it has already been processed...