1) add the Account to Cash Drawer that currently has the Prepayment (Deposit)
2) use the Prepayment (Deposit) as your tender type. The system will think you are creating a prepayment/overpayment
3) assign the Prepayment to the new Water account or different Class
4) process the payment
Below is a 1 min video on the process. (right-click > open in new Tab)
You'll see me change the Due Date in Cash Drawer because I had to trick the system into thinking there wasn't a balance on the account. The system will not let you create a Prepayment if there is a Balance on the account (unless you trick it!)