If the payment was already "billed" you can't change it unless you can undo the bill. Since it is old, that may be hard to do. There may be a way we can change the date behind the scenes but I don't know if that might cause other issues in the database. The other option is to duplicate the matter, export all time and expenses out of the existing matter and import into the new matter and then re-add the transactions and generate new bills. Then you could close the original matter to avoid having both in the system. Depending on the number of transactions and invoices this may be a lot of work and it is up to your firm to decide on the value. I would be happy to assist you if you want to go this route.