Moving an invoice without moving a product seems a rather unorthodox approach, and potentially a cause for accounting headaches. Would it not be better to raise a different invoice under the other client wanting to pay for another clients products/services and reference the original invoice number? As otherwise I can see a situation where you would be looking at the billing history of the client who owns the product and are going to see gaps in the billing history. Please provide your thoughts to help us understand this more fully.