Sounds like the extended soak in the bleach accelerated the process.
I`d also be concerned about any elastic that was bleached because chlorine bleach can
You might be able to get rid of the yellow stain by soaking the pants in another bleaching solution.
This time don`t use chlorine bleach,
use hydrogen peroxide or something like OxiClean that makes hydrogen peroxide when mixed with water.
Another alternative is to dye them a darker color to hide the yellow.
You can get some fabric dye from the grocery store for a couple dollars and dye the pants in a pot on your stove.
It`s tough to get precise colors,
and the yellow may still show through.
A new pair of baseball pants is only about $20 from a sporting goods store or online,
so keep that in mind while you`re looking to spend your time and money fixing the pair of yellowed pants.
I hope this helps you a lot.