I am having a lot of problems trying to do a certain thing. I'm trying to integrate a simple scene into Cycles from Blender Internal, and to do that, I have to append a version of the rig for my character that was designed for Cycles instead of BI. I really can't just go into Cycles and re-texture it. I can import the new rig, but I have to also be able to copy every single keyframe from the first version to the second. The copy and paste feature is simply not working. I can't just go into the dope sheet or graph editor, select everything and press Ctrl + C and paste the group of keyframes into the second rig's dope sheet or graph editor. It acts as if there's nothing in my clipboard at all. I have searched far and wide for something useful, but nothing works. Even the articles on this site which appear to have the same problem don't appear to be working for me. I am using version 2.73.
When an animation is created, Blender creates an object data called "action", that can be appended or linked to another object (in your case the object will be the rig). So you have to import in the new file the action from the old file. you can reach and rename your actions through the dopesheet, changing in the header of the window from "dopesheet" to "actions". Of course - to work properly - both rigs should have the same bones, same names, etc.. Check manuals about "actions" for more infos.
One thing I did to get Animation copied to the new Model is by Importing Both models in the same scene then delete one of the models, your animations and rig from the removed model will remain in the actions list, you can then switch to pose mode and simply select that animation and it will be applied to every bone on the scene, Though bones from the first Rig need to have same name as the 2nd Rig.