As I'm aware but need to confirm with your employer.You need to convert from TR to Non-B in Rayong not in Changwattana! Changwattana is for people who are working in BKK only!
Latest I heard the immigration office in Rayong denied to convert TR to Non-B so you may need to go to Lao and apply the Non-B from there.
How to get around in Rayong?
-The public van(really unsafe but most of people are using)