You can do that however you have to verify that first to your accountant if it's needed to change the date of the bill or follow the actual bill date of posting. Let's just make sure of this because Her Majesty's Revenue and Customs (HMRC) might be questioned why the dates of the transaction is not correct.
Also, the report for VAT exempt will base on your filing frequency that you've set up in QuickBooks Online.
You can consider checking these articles for more details about VAT exception in QuickBooks Online:
Personally, I don't like the idea of an Exception Report - or adjustments to VAT Return for that matter. I much prefer to have everything as simple as possible - so I will always enter missed invoices as though they were dated the 1st day of current VAT quarter (with a Memo to show the Actual Date).
As long as you aren't talking about £000s of VAT the VATman won't be too concerned.
(after all, this is VAT you should/could have reclaimed 3 months earlier)