A car's plate can legitimately change for a few ordinary reasons: a private or cherished plate being added, a private plate later being taken off and the car's original age-related plate reinstated, or occasionally an administrative correction.
On its own, a plate change isn't a red flag or a sign that something's being hidden - it's a routine, common thing to happen over a car's life. It's worth knowing about as one detail among many when you're looking into a car's history, but it isn't something to treat as suspicious by itself.