Motor vehicle VINs in North America use a check digit to detect VIN errors, which consequently makes it hard to modify a VIN without failing checks at time of purchase (such as when the VIN is looked up via CarFax or at the DMV). It's not cryptographically "hard" to modify it, but additional check digits up the difficulty (like appending a hash to the plaintext serial number).
The OCR-A character set used in the MICR numbers on checks makes it easy to spot a modification like the one in your story.
The OCR-A character set used in the MICR numbers on checks makes it easy to spot a modification like the one in your story.