I have a client in New Zealand who we just agreed I would start invoicing in Australian dollars - previously invoiced in NZD.
But now I’ve done that I note all of my old invoices have changed to being denominated in AUD, which I guess is going to mess the accounting up.
How should I handle this? Is my only option to create a completely new customer with the different currency, or is there some way of setting up dual currencies for a customer?
Yes - that’s the only option. You are basically opening new account for the customer. Mark the old customer account as inactive.