Congrats on your new client, Kate!
There's no need to delete their historical transactions. You can still balance their reconciliation even if they exist.
You have two options to reconcile them:
First option: You can undo all the reconciled transactions and start from the very beginning. This will give you correct reconciliation reports.
Second option: You can reconcile the historical transactions without touching the reconciled ones. However, you’ll have an incorrect ending balances on the reconciliation reports since we'll be using a different ending balance. This option is for the sake of reconciling the historical ones.
Here’s the explanation: You've already reconciled the later months. This means you will not use the statement's ending balance of the historical month that you'll be reconciling. Instead, you’ll use the ending balance of the last reconciled transaction in the bank register. Otherwise, you can't balance your reconciliation.
Let's say you've already reconciled May 2019, and you need to reconcile the April 2019 transactions. You’ll use the ending balance in May as the Ending Balance on the April reconciliation.
About the effect of deleting the transactions on the returns, the filed ones (marked as filed) will not be affected. However, the changes will be brought the to latest VAT return as exceptions. Therefore, I'd suggest not deleting them.
You can always go back to this thread if you need more help.
