I'll help with your questions about VAT reports in QuickBooks Online.
The amounts in the Exception Amount column represent transactions you have previously filed but were changed, deleted, or added. This is to make sure that no VAT collected (or credits earned) are missed or duplicated.
You can follow these steps to see the details of the transactions:
Go to Taxes and find the VAT period.
Click the dropdown arrow, then select View exception details. This will give you line-by-line, transaction-by-transaction details for your exceptions.
After changing the accounting method you must run the exception report manually on the old accounting method or reload it to get the correct amounts.