What to do? I'm trying to change my VAT number to my new registration but it doesn't update the date the new scheme started and just continues from the previous scheme.

Great having you join us here in the Community, @userxenia-sapanidi. I’d be glad to help you fix this concern about updating the date for the new scheme in QuickBooks Online. 

By default, the effective start date is a year after the end date of your previous return. The transactions that will be included in your next VAT return will be determined by the start date of the new scheme.

The system will track the changes in accounting methods and show them in the Open Items column on your next VAT return.

Also, please know that you may need to change the effective start date to include older transactions that fall out of this date.

If the issue persists, I suggest contacting our QuickBooks Online Support Team. They have the tools to find the root cause of your concern and find a fix. Here’s how to get in touch with them: 

1. Sign in to QuickBooks Online.
2. Select Help (?) at the top right.
3. In QB Assistant, enter the topic you need help with. You can also enter questions
4. Select Contact Us to reach out to our support team.
5. Choose a way to connect with us:

To learn more about it, please review this handy article about changing your accounting method in QBO: Change VAT schemes in QuickBooks Online.

In addition, I've got these helpful resources that’ll help you manage your VAT: 

• Set up and edit VAT settings, VAT codes, and VAT rates
• QuickBooks VAT codes
• Submit a VAT return via MTD to HMRC in QBO

Keep me posted if you still have questions or concerns about VAT in QBO. I'll be around for you. Have a great day!