All I was pointing out was that if an invoice has been raised then the date on the invoice is the point of sale and that determines the rate of VAT. So if an invoice has been raised the only way to get the new rate would be to issue a credit note and then re invoice after 1 December - which might invite questions if there is a VAT inspection.
If no actual invoice has been raised then there isn't a problem as it will be raised after 1 December and will attract 15%. However, the HMRC guidance does state that if an invoice is raised after 1 December for goods that have been paid for before 1 December then 17.5% applies so this might have to be applied to the deposit.