Correcting a post facto entry on a reconciled account

I changed an entry in a reconciled amount (original entry made 5 years ago) so that it became a transfer, the cash amount remaining as before. I now have a discrepancy in the reconciliation history of that account. (The account with the transferred sum cheerfully reconciles.) The current reconciliation attempt shows the 5 year old entry with a blue. I don't seem to be able to turn it into a green tick