Ko Chang isn't really far enough from Bangkok to warrant a night bus - it's 4-5 hours. Plus, it's an island, and the last ferry tourist ferry from the mainland to Ko Chang is at 5pm.
The car ferries run later, but I'm not sure if you can take one if you don't have a car! http://www.ko-chang.info/kohchanginformation/arrival.htm
Yeah, there IS a night bus leaves the EASTERN BUS TERIMAL at 2300. You are looking at a 5 hour trip - so you arrive in Trat town abt 0400. From what I've gathered there are some restaurant/bars at the bus station and down town where you can hang until the first sonthaew to the piers for Chang around 0600.
Been meaning to do this trip this way for years - maybe next visit.
car ferries take passengers too without cars
#5 Krabiman has been a member since 2/1/2011. Posts: 223