Very curved forehead - is it possible to shave the whole forehead, so it looks less prominent?

They say a picture is worth a thousand words. In addition, a face to face doctor's exam is likely worth more than words.  Many thing are possible in cosmetic surgery so without an exam the answers are all hypothetical. A prominent forehead you describe may mean you have a high forehead (like Rihanna).  

There are doctors who offer this tyhpe of sugery often plastic or maxillofacial surgeons which you should seek out

A picture would be helpful, but many times when patients complain to me about the curvature of their forehead it is secondary to a high hairline. This can be corrected with a hairline lowering procedure. 

If the curved appearance of your forehead is associated with a high hairline, bringing the hairline down will make the forehead look straighter. Pictures would help to see if this type of surgery would work for you.