Yes you can...but it's tricky
You can travel by land / sea from Trat into Cambodia and onward to Sihanoukville but it isn't easy.
There is one boat per day departing Koh Kong (Cambodia) at 08:00 am and onward to Sihanoukville.
You will need to leave Trat by 06:00 am to cross the border when it opens at 07:00 am to make it on this boat departing at 08:00 am. If you miss your connection you will either be stuck in Koh Kong, with limited accomodation possibilities or forced to take the gruelling mini-bus trek to Sihanoukville. The border is open from 07:00 am to 17:00 pm. You can buy your Visa to Cambodia at the border (with passport photo and US $) but I am told you can not get a Thai Visa going the other way...only a tourist card (if you qualify).
It is 1 - 1.5 hours from Trat to Hat Lek border crossing by minibus (100 baht). You will need to motorcycle taxi yourself from the border to the Koh Kong Pier for your onward trip to Sihanoukville. The boat (if you make it) will get you to Sihanoukville by about noon. You will need the remainder of the day to get to Phnom Phen. I know of no flights that will get you from Sihanoukville to Siem Reap. I'm not sure who flies into Siem Reap from anywhere in Thailand except Bangkok. If there really is a flight from Koh Chang to Siem Reap that certainly would make your plans easier.
This isn't a trip for just anyone. It's a long day with unpredictable sea conditions and terrible road conditions. I can't see making the trip if you have any more luggage than a single backpack.
Good Luck!
Surfcrest
For more information see:
http://vngold.com/kohkong/overland.html