Your most direct option would probably be to go from Surin to Sa Kaeo, and then from Sa Kaeo to Chanthaburi, and then to Trat where you can get a bus or songthaew to one of the piers.
Unless more direct buses are going than there were when I did the trip, expect to spend about 8-10 hours, or more, on the road. Just going to Trat from Prachinburi is a 7 hour ride.
I've done this journey in the opposite direction.I'd suggest the train to Korat.Tuk-tuk to the bus station and then bus to Chantaburi then bus to Trad for Koh Chang.It's going to take you the best part of the day so I suggest the first train out of Surin at 05:10 arrives Korat 08:20 so gives you time to get a tour bus to Chantaburi which I recall is over 5 hours.
#3 sayadian has been a member since 15/1/2008. Posts: 1,557