Up north we have to have to (or are at least meant to) register our number with the ASKC (Association of Scottish Kart Clubs). When I first went to register myself in late 2000 when I started racing, I didn't have any preferences. I got allocated #74 by "the boss" Carol Blanchard, and have never raced with anything ever since
Besides, if I did have to change, I'd need to get in touch with the powers that be at 10-10ths