Nice to have you in the Community, @at5. I can provide clarification on undoing a reconciliation.
When a user is looking at undoing an entire reconciliation, it's recommended to consider a few things prior to doing so. One of these is that the root cause of a reconcile issue is often fixable without having to undo an entire reconcile. I recommend the following article for additional things to consider, as well as steps for undoing a reconciliation should you choose to do so: How to Unreconcile a Transaction or Undo a Reconciliation.
Additionally, I suggest the following resource which contains multiple articles to assist with the reconcile process: Reconcile Hub.
These resources a good place to start when looking at undoing a reconcile.
Please let me know if I can be of additional assistance with this or anything else. Have a good day.