You can't just replace a grommet with a spring. There are some other parts that change, too. The best way to track down all of those parts is just to buy a spring-based Wico in the first place- if you can find one. Most, if not all, of the microswitch-based Wicos have the spring. I don't know that I've ever seen a leafswitch Wico with the spring.
BTW- somebody (Paige, I think) swears by the spring-based Wicos, but only if the spring is swapped out for the Happ P360 "Heavy Duty" spring.