Well, depends on how much work you want to do...
You can always unwrap in your 3D Package of choice, and then manually adjust the unwrapping later. Meaning, pulling the vertices into the right place until everything fits nicely.
A lot of work? Yes. But as long as the two objects have a similar topology, and the seams at similar places (as opposed to the same topology with completly different seams, which will never fit nicely without any texture stretching), you will be always able to make it fit.
Now, in your example texture stretching should be pretty fine. If pieces are more distorted, that might turn into a problem (Like trying to fit pants from a dwarf to a tall, skinny guy).
Then there is the problem with baked normals... these normalmaps are usually baked in correspondance with the normals of the polygons they are on, as well as the normals of the vertices surrounding the polygon. Depending on the tool used to bake them, applying the normal map to a different model might show artefacts at some points, because while the unwrapped topology is spot on, the shading generated by the vertex normals might differ, thus the normal map no longer fits perfectly.
Not huge artefacts you can expect here, but might still be problem. As said, depends on the tool you use for baking and what kind of normal maps we are talking about.
In 3d coat at least, there is a way to "remap" a texture baked/painted for one mesh to another one with a different UV map. I cannot find the video that explains the technique right now, and never got around to try it myself, but just as an example that there might be tools out there that automate tedious things like this for you, you just need to keep looking for them.
