Can I change the Category/Chart of Account Number in an expense payment in a previous month which has been reconciled in checking?

Thank you for posting here in the Community, userconnie.

 Yes, you can change the Chart of Account number and I’ll make sure that you can change it. No worries, this will not affect your transaction.

 
Here's how:
 

1.   Click Accounting from the left menu, then select Chart of Accounts.
2.   Select the desired bank account, then click View register. 
3.   Select the transaction.
4.   Select a category from the drop-down list.
5.   Once done, click Save. 

If you need further assistance in changing the reconcile status, please check this article

•  How to unreconcile a transaction or undo a reconciliation.

 Please do get back on this thread if you need further assistance in changing the Chart of Account number. I'm always here to help you.