What's the Name of That Book??? discussion

I am trying to find a book that I read in the 80's. This book is set in space. I think it was about an expedition to Neptune or Pluto. The scientist decide to use kids or teens, not sure which, because they would be changing their DNA to create bodies that could withstand the trip. The kids end up in bodies that are completely different. When they get to the planet, they find that it is not empty. There is some civilization, and the civilization is more advanced, and is creating new life to seed the universe. I seem to remember that the majority of the scientist were bad, but 2 were very nice to the kids and join them in the exploration. Any help would be appreciated!

Did they start out on Earth, or the entire book is set in space? Do you remember anything about the cover?

No response, moving to Abandoned.