To clarify, once you have dragged in the SRD (generic) version to a game, that becomes the resident copy for that creature. Dragging in the MM version later will only result in a new copy of the SRD version being created on the tabletop. This is intentional behavior and designed to protect your personal edits and home brew. For instance, if you gave your local copy of "goblin" axes instead of scimitars, you would not want a subsequent compendium drag to overwrite that. Hence, Gauss' advice to remove all copies of a monster from the game's journal tab before importing the desired one.