Hello there, @ar12.
When you add an attachment to your attaching files, they will do nothing to your reconciliation.
However, if you change your VAT code to a transaction that has already been marked on your VAT return, it will cause an exception report to the current Vat return.
To know more about VAT exceptions and reports, here's an article that you can refer to.
If you have other questions, you can always visit us back here. Have a good day!