Best answer by woz
you can't normally change the date of the first payment, it comes out the date you switch. You will be able to change the date thereafter.
Assuming no issues with your switch it normally takes 17 days to switch. I mention that because normally I'd suggest cancelling the switch and re-applying, count forward 17 days will be your first DD.
Did you switch after the price increase was announced? If you applied to switch before the price increase it means your prices are fixed 'till the 15th Jan, and if that's the case with your switch I wouldn't cancel. If after the announcement cancel the switch and re-apply (you may have to wait a couple of days to re-apply)
You could ask through Help in the app (type "message the team" and ask) but even if they were to agree there is no guarantee it could be actioned in time.
Sorry I can't offer a more certain solution, but also bear in mind if you're switching from another provider they will also continue to bill you until the bill is finalised and that can take up to 6 weeks, so if you're unable to pay PP you may find yourself in further difficulty. (you'll need to contact your previous provider about that and ask if they can reduce the DD)