Once the astronauts get on the ship, it starts to change to fit the human beings needs. The gravity and the atmosphere change, the structure of the ship's instruments changes as well etc. Finally, the ship becomes fully useful for human beings, because all it needs is to trap the astronauts and use them for fulfilling its goal - travelling the Universe. I read this book some time about 1981/1982. The book's title may be "The Ship" or "Relic", I'm not sure, but I can't find any book titled like this and with the story I wrote above.