How to change customers on an invoice
If the invoice has a job, change the job customer, then go to the invoice and confirm it has changed the invoice over as well.
If the invoice doesn't have a linked job then it gets a bit tricky.
Open up the invoice needing the customer change.
Look in the URL, at the end will be like .....&invoice_id=123
add this bit to the end of the URL: &change_customer
so it looks something like this ....&invoice_id=123&change_customer
then press enter.
This will enable the "Customer" drop down box on the left.
Pick the new customer and save.