Yellow gold is an alloy of gold and copper, white gold is an alloy of gold, and a silverish metal, like nickel, or palladium. So you can't convert a yellow gold ring to white gold.

Plating may be an option. (I'm a metallurgist, not a jeweler, so you'll have to ask them, but we plate aircraft engines parts all the time) I'd say your best bet would be Rhodium, Platinum, or Palladium plating.

Just take it to a jeweler and ask if it can be Rhodium, Platinum, or Palladium plated... (all are precious noble metals that won't tarnish) it should look just like white gold after that. You might have to have it redone every couple of years though, as thin coatings can get scratched.

Not sure how much they'd charge for it.