Sci-fi, shapeshifting protagonist

I just remembered a book I read once, but I can't remember it's name.
Plot revolves around a genetically engineered man, which is built to explore alien planets by making it shape shift into a dominant species of that planet. Making it simpler to observe those aliens.

Maybe Darkchild by Sidney Van Scyoc.

Nope, that's not it.

Are you sure that it was a genetically engineered man? Julie E. Czerneda has a series with a few similar points. An alien species capable of shape shifting into any creature (matched down to the genetic level) that uses the ability to observe and record details of every species they encounter. First book is Beholder's Eye. Probably not what you are looking for if the details you remember are solid but it's a good book. You might enjoy it while you look for the right one.

I am sure. That's not that book.

Sounds vaguely like Consider Phlebas by Iain M. Banks - long time since I read that though, so not sure if the plot's exactly right.

In Phlebas it wasn't really a shapeshifter (except for changing gender at will etc.), and the aliens knew he was Culture... (I just read it a month ago).

For me it sounds vaguely like Legacy by Greg Bear, but I am not sure if Olmi was able to shapeshift. I do know that the aliens didn't know he was from The Way though.

Also, another Culture novel, The State of The Art has Culture aliens visiting earth unrecognized... but that wasn't shapeshifting either....

oooh, there is a book on the edge of my memory with shapeshifters and that they were forbidden to do so in a certain context...

To make things clear. There are no aliens in this book, everything happens on earth. The character is engineered and it was some time in the past used with couple of species. The individual of that species is rather merged with the man, than copied. They are sort of multiple personalities with a real option of changing appearance at will.
The protagonist is now on earth, because something went bad with the project. Though I am not sure what the governments stance on that.
The man is now getting back memories of the person he was cloned from.

Just to tell you that, that I found what I was looking for - The Werewolf Principle

yeah , thats the name!