chain size?

I would like to convert the chain and gears on my sons cr85b. I was told that there are conversion gears made that step chain size up to 428 from the smaller 420. Question is what is the difference? Wider or longer? Both? Do I just change the front sprocket? Please help as I am getting conflicting imformation from the local shops.

Thanks Cody305. The reason I wanted the wider gear was the output shaft spines seem to be wearing out. I have only four races left in the season and the bike is being retired. Hoping to not have to split the cases to reploace the shaft.