can I change all materials in a scene to glass for example, all at once ?

[hugo]

instead of one at a time, see attached pic

[KeyShot]

Yes you can. Just right click on the group in the scene tree for which you want to change the shader and then select change shader. The shader will automatically inherit the attributes from the existing materials. You can also assign just one material by dragging it onto the scene tree.

[hugo]

I tried your solution prior to asking my original question, .....sorry cant screen capture right click dialog menu
the right click dialog menu > material >link material
                                                           >unlink material
                                                           >change material type > change to "glass"

all materials are changed to clear glass, but don't retain the RGB colors in that group 

[KeyShot]

Glass is a bit special when it comes to colors. We can look into it.