All of the xml for your savegame is stored in the save, not the datazip so editing your datazip wouldn't effect saves.

In the olden days it took a lot of time to read in all the games xml (not literal i/o time, but translating all the strings to game entities).  So the datazip was created to store all that xml in a form that the game could load more quickly.  If your xml changes the datazip is rebuilt, making load times longer when the xml changes, but much faster for the 99% of the time where the xml doesn't change.

But when you save we write out all the xml into the game save.  That way if a patch or something changes your xml your game save is still valid.  It isn't 100% since we change the source code to act differently and if you are loading saves from old versions you get old xml with new code that can cause wonky problems.  But its better than blowing up because you have an ability on a unit that doesnt exist in the patched version.