The Service functionality should be extended to be consistent with Sales functionality, currently from Posted Sales Invoices you can Correct, Cancel and Create Corrective Credit Memo. Similar functionality should be added to Posted Service Invoices to allow easy correction of those invoices.

This might not be practical for Service Contract Invoices so I would suggest the following:

For Service Contract Invoices this should also include the rolling back of the Service Contract, IE if the Service Invoice is cancelled then the Service Contract should revert to the same dates that it had prior to the generation of the Invoice, this would then allow corrections to be made and a new invoice generated.