I was wondering if I can cast a polymorph spell just to change the shape and the aspect of the yet polymorphed being..... or do I have to wait until the polymorph quits ???

If it's available.... then you know how many possibilities have this ??? My polymorphed rogue friend was in a random house, just to sneakily ""steal... ahemmm"", and me on the roof of a house in the vaccinity, taking a look if my polymorphed friend comes out. Then, if my friend is chased by the city guards, I can cast Polymorph over my friend to change the aspect and shape of him ( to stop being chased )... hahahahaha.

Is this possible ???

Because of the way spells stack, I believe this is possible. The first polymorph effect would fade as soon as you cast it again.

Of course, in the situation described, you could just drop concentration on the first polymorph effect, assuming you cast the spell, and he'd be in an unfamiliar form without you having to spend another 4th-level slot. :)

A second polymorph on the same target overrides the first polymorph as if that polymorph was ended. 

If you are the caster, you can just drop concentration to remove the polymorph as EddyJ suggested. 

Ok..... thnxs