You'd have to import the 1.3 reference smd triangles onto the skeleton of the 1.2 reference smd skeleton and reassign all the verticies to the correct bones, then recompile the mdl file with textures. In other words, not a 5 minute job if you want your characters to deform correctly. It's also likely that the 1.2 skeletons will not fit the 1.3 models perfectly, so it is guaranteed you will get some weird deformations.