If that's the case, you could have chicks with yellow legs in two generations.
Breed the male with yellow legs to the hens with white legs.
Take chicks from that mating and breed them to yellow-legged mates (50% of chicks should have yellow legs) or breed them chicks to each other (25% of chicks should have yellow legs). So if you hatch 2x or 4x the number of chicks you normally would, you can sell or butcher the white legs as soon as you are sure which ones they are, and have about the right number of yellow-leg ones to screen as possible future breeders.
Whether it's worth doing would depend on how important the leg color is to you.