Hello there, if you’re a photoshop wizard, could you take a minute to enlighten a self-learning half-newbie ? :D

This is about pngs in indexed colors bugging me, in a nutshell.

To describe it simply, I have a problem with pngs, when their color set is “indexed colors”.
When I open them in Photoshop (cs5) :
– I cannot duplicate layers, create layers, it’s like mono-layered
– when I select a zone and then I attempt to perform a clone stamp inside this zone, the clone stamp isn’t “previewing” (in the circle-shaped mouse pointer) what it is going to be drawn

The problems don’t end here ! Because of this, when I want to do something with that kind of png, I convert them to grayscale or RGB. But, to my horror, this conversion changes the picture !
Look, compare an original png, still in “indexed colors”, with the grayscale-converted version.

I suppose there must be an explanation.
I hope there is a solution to avoid the picture change when converting out of indexed colors.
But my google-fu didn’t allow me to find neither a solution nor an explanation :-|

So please, if the reply is obvious for you, you’re welcome to flame me or ridicule me (as a friend once yelled after far too much alcohol : “attach me with manacles to the radiator and speak to me in German !!”, maybe that is horrible), as long as you have a valid reply ;)