How to edit weighpainted objects

This problem really drives me mad:( I have got this character, a mesh on an armature, and the whole thing weightpainted. So far, so good! But this character is more like a prototype, a very simple model, I want to give him some new clothes, but I havent designed it yet! That is a typical situation for me. When I then design lets say a nicelooking shirt with drapings etc., I want to put it “on” the character instead of the prototype shirt he was wearing and that was weightpainted. I remove the old shirt and places the new one.
But then the problems start. At first glance, it looks nice, but when I start weightpainting again just a bit, the whole mesh deforms and crazy stuff pubs up!
How to do this? I have tried of course alt R and alt G, so that is not the problem! Seems to me that the old mesh kind of “reject” the new part of the mesh, eg a shirt…

This is very difficult to explain, does anyone get it? :eyebrowlift2:

And by the way:
If I make an armature with various actions, will I then be able to change the weightpainting of this armature and thereby all the actions done hitherto? eg. : I first made the weightpainting around hands and fingers a bit impresise. Then I recorded a number of actions in the action panel. will I later on in the project be able to change alle these actions slightly if I e.g make a more precise weightpainting of the area between the fingers?

To solve your first problem, it would be helpful if you posted the .blend file.

As for your second question about the actions, yes you can re-weight paint the mesh and it will affect all of your actions. Actions are just keyframes movements of the armature, innately they have no relation to the mesh.