Are the posts only bowed in one principal axis?
that means, when viewed while facing one side it looks straight, when
viewed from 90 degrees off the first view , it looks bowed
if only bowed about one principal axis (like a bowed yard stick)
you COULD straighten it but IMO it would less time, effort & material
cost to replace them
Do you have an appropriate steel member to use as a strongback? and a
bunch of clamps?
a better question is why are they bowed? are they over loaded?