Ivy,
WELCOME TO THE AWS WELDING FORUM!!Legally, ethically, or technically??
It comes down to wording and many other perspectives. You said "can". Sure, they CAN. They have a pen or computer file of the report and CAN do anything.
What are the repercussions if any? Total liability in every legal aspect. Falsifying a report that may be a legal document, all depends upon who the NDE was done for, who reports were submitted to, what the purpose of the NDE and report was, what code was applicable for the work, what the contract documents said about inspections and/or special inspections, and so much more.
Bottom line, only a fool would change someone else's report. Even if a company wants their techs to submit reports to the office only and then the office reviews them prior to sending them out, if there is a mistake they need to send it back to the inspector/technician for correction. If it ever went to court and the inspector walks in and takes the stand with a report that differs from what is submitted by the owner/engineer/local building authority as well as the inspection agency then things are going south really fast. And people who have changed anything just lost all respect as a viable, truthful, witness.
He Is In Control, Have a Great Day, Brent