how to transfer funds to another occupied unit?

In the move out process, Pro-rate if necessary. Under "refund" at the bottom, apply to another unit. When you process the move out, it will ask which unit to apply funds to (It will only let you chose another unit rented by the same person).

After the move out is complete, go into the payment screen for the remaining unit and apply the 'refund' to the charges. You may need to create the next months charges first.