The Only Alien on the Planet question

So, The Only Alien on the Planet is my favorite book and I have read it at least 6 or 7 times. I gifted my copy to my friend for Christmas and bought myself a new one. The new one has a different cover, but I assumed it was the same book (same title and author). I started reading it today and it's different!!! It's the same story, but a lot of the dialogue has changed which makes me so sad. Has anyone else noticed that there are two different versions and know why the author changed it?