Just changing to a new numbering system when they run out.
I wonder what the practical reason is for not just allowing full alphanumeric number plates. Each digit would then have 32 possibilities (I, O, Z, and S should be avoided to prevent confusion with 1, 0, 2, and 5). This gives 34.36 billion possible number plates which seems sufficient for at least the next couple years.
Or put the letters and numerals in set spots, ex. ABC-123, next move onto 123-ABC once you’re done with the first bazillion combinations , AB3-12C, etc.
That way you can tell your 1 isn’t an I because it’s not in the right spot.
UK includes the year in it, so it shouldn’t ever really run out. Ok I guess eventually it will loop but I expect most will be available for reuse by then.
One issue could be if more cars are registered than the digits would make available for that year but you would probably just design it in a way there is significantly more space than you are ever realistically going to need.
We actually did loop in Denmark a decade or so ago. It was quite easy to guess the production year of the car by just looking at the first two letters. It was a bit trippy seeing new cars with "AA 11 111” all of a sudden when we ran out of ZZ’s.
