Yes, you can change the customer a project belongs to up till a certain point. Projects with timesheet entries that are already on an invoice cannot have their customer updated. This is to ensure the invoice currency remains consistent with the currency of the customer.
To change a customer, click the Project Settings cog next to the project title and choose the new customer from the drop-down list.
Feel free to reach out to our support team via chat or firstname.lastname@example.org in case you need further assistance.