Something caused Brueckner to do this. I don't know what it was, but it sounds like panic to me as an almost immediate afterthought.
The new registered owner would have very quickly been out of any frame as he actually was in Germany.
This was the beginning of the whole thing falling apart as far as I can see. One small and probably unnecessary mistake. Unfortunately, The PJ didn't spot any of this, including his Jaguar. It is only now that it has become relevant by what Brueckner did do.
However, Brueckner did do this and he must have had a reason. It is logged and on record so cannot be argued otherwise.
Hindsight is an exact science but there was a time when a little foresight on behalf of the investigators might have resolved the situation at the time.
I think the failure to check the phone pings at the time is extraordinary bearing in mind the amount of attention concentrated on those of the McCann party. Maybe they did; because it remains a bit of a mystery why the PJ attempted to speak to Brueckner.
So it had to be that or they were acting on intelligence which we know nothing about.
He definitely had a reason to change the car registration and I would not rely on the timing being coincidental.