Getting a Chang map on the island isn't a problem. They are handed out in hotels and offered in restaurants. If you've lost your map, you'll have no trouble getting one from the local tourist workers. Download a map of Chang to your smartphone, and all the secrets of the island will be available to you. They even offer a map of Chang Island on the ferry from the mainland.
And if you don't know where you're going to stay, a map of Chang with hotels will certainly come in handy. There's a similar map on the island's official tourism website, here. Although Koh Chang is quite small, but it keeps a lot of secrets. And those who like long walks will have something to do. All the more reason to keep a map with you at all times!