Change mesh when animations already exist

Hi everybody!

I created my character model with blender v2.67a, added an armature, did all the weight painting stuff and finally added a couple of animations. I use this model as the player-character for a game that I create with XNA. After seemingly endless hours I can now run with my character from left to right and vice versa in my XNA game environment.

Now I want to give my model (the existing meshes) a bit more details but this is not as unproblematic as I thought it would be. For example, when I slightly change the position of 1 vertex of the right upper leg in Edit Mode (of the T-Pose) and then I select an existing animation in the Action Editor, the whole right upper leg will be missplaced (it will be placed under the right shoe on the z-axis).

The other problem is that from now on the 3D view will always display the first of my animations, no matter which one i select in the Action Editor.

So the question is: How exactly can I change the mesh without to mess things up?