EDIT: Replaced the picture with a new version based on Tim's feedback ... darkened the guage a bit to match the body ...
If you try the same modifications on a duplicate layer, then add a black layer mask and then paint in just the bits you want to change with a fine white paintbrush in Photoshop, you get the advantage of local changes, leaving the good bits of the image unchanged.
If you keep a version with un-flattened layers, then you can go on tweaking without having to go back to the original each time.
I have to admit that I was very uncertain of this until inspired by Jack & Sue Drafahl's book, and now I'm a big enthusiast for non-destructive editing.
Makes up for not having a D700...
Tim
B)