There were very few changes to the creatures between C&T/C&T II and C&M. Except for the wholesale replacement of the pseudo-Lovecraftian weirdo category (Elder Worms -> Agothu, or something like that), most of the alterations appear to be errors at least as often as updates. The format was changed to the far less compact C&T III version, with some uninspired prose padding to fill things out. I consider the C&T/C&T II monster entries to be an all-around superior version. Even in simple updating to RMSS core rules, C&M fails in several places. Treasures saw more change, with some attempt to actually match the items to the item-creation rules. Treasure Companion still contains items that violate its own rules, but it is much closer than RM2 ever was to coherent treatment of magical items.
You will have to make some adjustments between the versions, but these are few enough. If you have the appropriate RM2 books, you can leave some creatures (or items) with their original abilities instead of falling back on the alternative abilities (and C&T II usually gave a core power to replace powers that relied on a companion book that might not be available).