Ko Chang --> Cambodia?

The Thai buses are better and more honest then the Cambodian buses, so my advice is go to Rayong and take a bus from there to Aranyaprathet near the border and spend the night there. Next day over the border and on to Siem Reap.
The B300 bus direct to Siam Reap sounds like it will be an ordeal, why knock yourself out?
Or, skip Siem Reap anyway, its just a tourist rip off everywhere - you can do it on another trip.
Go to Sihanoukville and spend your time there. It is a good time to be there.

there are numerous options and most are easy to do but with some problems on the way

If you say where you want to go in Cambodia first and if you want to stop on the way then can give better advice

However please ignore the advice of my friend ripoffallerter as it would be madness to go to Rayong from Koh Chang it is miles out of your way. There is a regular bus from Chanthaburi direct to Aranyaphatet (destination Buriram) and there are numerous buses from Chanthaburi (and some from Trat) to SaKeow where you can pick up bus to Aranyaphatet (and a mini van from Chanthaburi to SaKeow

Siem Reap a rip off but recommends Sihanoukville!!

I did this trip recently in two days to Sihanoukville, First day I crossed over and spend the night in Koh Kong and took the 8 am big bus to Sihanoukville. It was a comfortable 4 hour ride on the big bus, and Koh Kong was an interesting little place to spend a night. Breaking the trip in two made it pleasant, instead of being a pain.
I would not go overland between Siem Reap and Sihanoukville, you could break your trip in PP or fly instead.
Just read up on the scams and get the online visa and you won't have any trouble at all.

Yep it is usually best to spread the journey from Koh Chang to Sihanoukville over the two days as I do not think there is an afternoon bus from Koh Kong (unless new) BUT it is not unusual to find a taxi or van that has come from Sihanoukville looking for passengers for the return and if you can avoid the border mafia they will give you a good price

For the trip to SHV forum2233's recent report is very detailed and recent. You really should read it if you go for that route.
Bigal pointed out that the buses run from Chantaburi to Aran in Sakeow province. They actually stop at Aran bus station from there it is a 6 km motorbike or tuktuk ride to the border. Look for the Buriram bus at Chantaburi. Buriram is the line's terminus. Third class fan and 2nd class aircon buses service the route. Fan is no problem as the route runs thru lush forests and you get a nice breeze. Very friendly staff, working people who have nothing to do with the tourist industry. Nice crowd.

If you want to go by Pailin you need to get out about an hour north of Chantaburi at a crossing. The bus drivers know the spot and if they have problem to understand you people in Chantaburi bus station will help out. Cross the road as the songtaew to the border leaves there. You need to make it clear that you want to go to the border since driver goes to a nearby village and needs to make a small loop for you.
ALTERNATIVE: Aircon casino van to the border casino. Costs a little bit more but probably worthwhile.
After the border: Motorbike to Pailin. From Pailin to Battambang vans or public toyota taxis. I like the toyotas as they are likely to give you some 10-15 min along the way to take a quick look at three Khmer prangs right next to the road (100 meters off).
If the water levels allow, take the boat to SR, in case you have not done it so far. Very impressive, make sure batteries and memory chips are fit for the workload ahead.

I prefer the boat trip Battambang to SR and not the other way around. You see the river in the morning when it is less hot and have cooler temperatures at the lake later on. Make sure to bring a hat for the boat and the temples in SR.

Finally, I doubt that there is a direct bus from Koh Chang to SR. There is a direct ticket involving different companies and, I m prepared to place a bet, a series of scams. The latter include shitty restaurants, extra charges at the border, and a host of annoyances in SR on arrival. It is the same story almost anywhere in Thailand were private bus companies offer such services. Go to Trat bus station or to the one in Chantaburi and get a regular bus (in Trat it might unfortunately be a regular van).

Go to Trat bus station or to the one in Chantaburi and get a regular bus (in Trat it might unfortunately be a regular van

if a choice when you get off the ferry go to Chanthaburi as all buses from Trat detour through.

Yes well I did not know of buses from Chantaburi, only from Rayong.
Anyway, if you do travel from Ko Chang to Trat or Chantaburi, you will leave yourself open for a likely tourist expensive fare, and then you will have to buy another fare from Chantaburi or Trat.
If you have a B300 fare already available to Siem Reap from Ko Chang, it might be an option to take that with your bags inside the bus, and leave the bus when you have had enough at a likely place to board another bus.
If deciding on Koh Kong, make it by noon and there is a bus to Sihanoukville, however it leaves you on the highway to wait maybe two hours for a bus from Phnom Penh to Sihanoukville.

Please just how do you think the OP is going to get to Rayong from Koh Chang

yes via Trat or better still Chanthaburi (where there is a direct mini van) so same scams on that part not doubt. And from Trat or Chanthaburi there are no scams on the bus they always sell the ticket at the right price - of course with your luck....