Generally, changes should be applied to all material zones regardless whether the zone needs to be changed or not.  This is just a matter of consistency throughout all the body zones for appearance sake.  This should not have any bearing on any applied morphs unless, of course, using external binary morphs is somehow getting corrupted in the save as vilters is stating.  However, there is not enough practical information to determine if this is issue. 

• Is the M4 morphed character a saved model in your Library, or
• Is it a base M4 which you are applying morphs, then attempting to change the skin textures.

If it is the latter, attempt to apply the textures to a newly morphed M4 and save.  Bring him back into Poser and note if anything changed.  As another alternative, bring your established character onto the stage, apply the textures, and save back under a different name and Library location.  Reboot Poser, then bring him back into the scene.  Do this under the assumption that the originally save character was not corrupted initially.

And, if you are using External Binary Morphs, turn that off completely and repeat either of the suggestions stated above.