Good day, richardfri.
A description or memo won't affect your reconciliation. You can add or edit them even if the transactions were reconciled already.
Simply open the transaction, enter a description or memo, then click Save. If there's a warning, just click Yes or OK.
You may want to read this article for more details: Reconcile an account in QuickBooks Online.
Keep on posting if you have more questions. Stay safe and healthy!
