Honestly, quite a few.
Across much of Europe, an issued invoice is treated as an accounting document that must remain traceable for VAT and audit purposes. You generally should not be able to overwrite it in a way that destroys the original record, especially when the customer may already have booked the invoice and claimed input VAT from it.
If an issued invoice is wrong, the normal process is to credit or reverse it and issue a corrected invoice.
That said, I think you are focusing on the accounting mechanism rather than the actual usability problem. WHMCS should make this process almost invisible:
You click Edit invoice, make the required changes, and WHMCS automatically credits the original invoice and creates a corrected replacement. The customer receives the corrected invoice and pays it as normal.
There is no reason the user should have to manually cancel, recreate and resend everything. Preserving an audit trail is reasonable; making the workflow unnecessarily cumbersome is not.