There usually isn't any way to adjust foam. Either the saddle fits or it doesn't. There are some "hybrid" panels that are a layer or two of foam and the rest is flocks, and those, you can adjust. CAIR can be shimmed, a little, but it's either a fits-or-doesn't option. With either system, the tree shape and width still have to be right (or "close enough") for the horse.
Foam panels can harden or deteriorate with age. You can have a saddle replace them with flocked panels, which requires a whole new panel, and the cost I've seen posted for that is about $275 or so. I think stripping and reflocking a flocked saddle with all-new runs about the same.