"What happens when you get to ZZ999?"
Don't you mean IF? Not sure that recent procurement policies would make that eventuality something worth considering.
Although, it seems that units can now choose "personalised" registrations (like 45 Sqn and 32 Sqn have managed) if this article is anything to go by :-
http://www.wolverhamptonaviationgrou...f_sequence.htm
Wonder if you have to pay MoD like you would to DVLA for a personalised car plate? How much for ZZ617?