There is a bus directly from Burri Ram to Chantaburi, or you can go from Prakhon chai bus station to Long Sang and chact the bus there it is 60 km shorter than Burriram.. From chantburry,to Trat eller koh chang you can find a bus fra the busstation or wait. It takes about 6 hours from Prachon chai to Chantaburi, and 1-2 hours to a boat. to Cho Chang.
The bus used to be every 40 minutes between Buriram and Chanthaburi no idea about cost but not much and easy onward connection to Trat. If you want to save time you could ask the bus driver to drop you on the main road where the bus turns into Chanthaburi and pick up the Trat bus there. Train not even an option