When you convert to a Cash VAT company in QuickBooks Online, you will have to manually change the open invoices to include VAT correctly, this includes removing a VAT item line created during the migration. Please ensure you enable VAT in your company.
Please be aware you only need to do this for open invoices as the initial information should come from your previous provider's VAT report you saved before converting your data.
Update the line item to include VAT
Select and view the first open invoice. We will have to change each line item to include VAT correctly. Please ensure the amounts are exclusive of tax and select the correct VAT code/rate for each line item. Once you have changed all the items in the invoice you can remove the VAT line item created during the migration process. Select Save & Close. Repeat for all open invoices.
This makes sure the VAT actually shows up when you do your first VAT report in QuickBooks Online.