can't be bothered to quote everyone, I simply printed off a 0 in the same font and size as the passport, cut it out and stuck it on over the 1 in 1991 with a tiny bit of selotape. Obviously i did it to a decent standard, and it's been working great. It even stood up to someone shining the little blue light on it for about ten seconds. It helps that I'm 6'4" so they don't look too closely most of the time.
Best bit of all is that if you need to use it properly to go on holiday you can just take the selotape off and there's nothing permanent unlike using a pen.