Replacing particle board subfloor with plywood
If I were you I'd keep those risers there. You don't know if they're glued on the bottom and you might damage the plywood underneath. Sand those risers to get rid of the glued particle board remaining on them. Then replace the particle board with plywood of the same thickness and make sure you glue it to the risers in addition to using a fastener. Fastener does all the structural work and the glue prevents squeaky floors.