Preface: I am an architect based out of the midwest, so I don’t have much direct experience with crawlspace rennovations or construction specifically, but I have worked with lots of wood construction and delt with water infiltration regularly.
Absent more detailed information (i see your other comment describing the wood piers and wood rot, but there’s still some ambiguity), I think it’s worth pointing out that concrete foundations are not the only way to prevent future wood rot.
Typically you see rot in conditions where:
There has been flooding under the house
there is moisture seeping up through the ground underneath the crawlspace
there is humidity in the air that is saturating the wood
The reason you’re seeing wood rot under your house is not because you have wood foundations, it’s because moister is not being properly controlled. Having not had experience on this particular situation before, I couldn’t tell you how you could approach replacing a wood foundation with a concrete one without demolishing the existing structure. It’s possible it could be done, but it’s almost certainly more expensive and won’t solve the water infiltration issue anyway.
I would reccomend determining the source of the water first.
If there’s flooding, install a drainage system (or repair/upgrade the existing one)
If there’s water seeping in from the ground, replace/install a vapor barrier
If it’s a humid environment and it’s not being controlled, mitigate the humidity by insulating, sealing, and ventilating/dehumidifying the space
You should not have problems with wood piers/posts if properly done. If you’re still determined not to use wood piers, I would ask your contractor about using CMU instead of the wood posts. keep in mind, though, that this won’t solve the issue of water presence and you would still likely have problems in the future if you don’t mitigate it.
