"The problem now is that the reconciliation for that month is wrong"
You don't have to Undo Reconciliations and redo them.
When you know the data still is correct, no matter how you changed transactions (one check should have been a Bill Payment check, or two Checks should be one check, or whatever), it should reconcile again just fine.
This is called Interim Rec or Mini-Rec. Use the most recent Ending balance and re-reconcile, ignore the beginning balance you see. That is the Net of all cleared transactions and you know you affected that Status. Checkmark them and you should end with the same ending balance as last time, if you really made no Different and just reworked the transactions.