https://s-media-cache-ak0.pinimg.com/originals/37/8f/52/378f529e7629063e1c0db3764360d2bd.jpg

Or did she simply lose the baby fat and got better eye make-up?

I don't see it. The only thing that might look just a little bit different is her nose, but you can't really tell because it was shot at an odd angle in this picture.

I'm inclined to say no, just because plastic surgery wasn't all that common pre-1964 and she was already quite pretty without it. Boy, does she look exactly like her dad!

Elizabeth Montgomery had a serious phobia of needles and medical procedures, the last thing she would have ever done was have plastic surgery.

No she did not have any plastic surgery before, during or after Bewitched!

Doesn't seem to be work done before Bewitched that I can tell. But she clearly had a facelift sometime in the 1980s (which turned out very good) and she had her teeth done (most likely crowns) which eliminated that slightly crooked front tooth. The teeth change is very noticeable as it altered her smile quite a lot.