If these are Polish registration plates the act of swapping them is not unusual. In many foreign countries registration plates are per registration document (ie. that particular combination of owner and car and serial of V5 equivalent doc) not like British plates - given for lifetime of the car regardless of who buys it later. So the act of swapping would just mean the ownership of the car has changed (for example from father to son).
Also, maybe he just got plates with EU flag for trip abroad (legal requirement) or got himself a personalised plate?
Also, maybe he just got plates with EU flag for trip abroad (legal requirement) or got himself a personalised plate?

I said its been posted on here.