Depending on the age, aren't there different VINs and chassis numbers? They're basically the same number but with different quantities of digits.

Maybe there's something obvious, like the 'new' plate has screws instead of rivets, or it has a custom one-off stamping plate instead of a machine-stamped one.

There are several threads with people having issues registering there cars because the 'VIN' doesn't match everywhere or it doesn't have enough digits to get entered into the computer.