My client de-registered for VAT in February this year and re-registered in July this year. Quickbooks won't accept the new registration effective date.

Hello there, h-goring-mcphers.

I understand that the issue with entering the new registration date has caused inconvenience for your client in filing their tax returns via QuickBooks Online (QBO). 

To better assist you, I need to clarify a few things. Did you receive any error messages when trying to enter the new registration date? If so, please provide me with the exact message. Also, may I know the specific user type or access of your client?

 This information can help me provide more tailored solutions to the issue. Thank you for providing more details. I look forward to working with you again, h-goring-mcphers. Have a good one.