Are you're hotels booked Already?
Assuming if you booked with miles, I would have booked an open jaw itinerary with Koh Samui being my destination and flying to there first because thai airways has two flights daily from Bangkok and Koh Samui.
Then book two one way paid tickets on Bangkok Airways from Koh Samui to Siem Rep and to Bangkok.
You can still change you award itinerary as you're not flying until august.