I have been on a non immigrant 'O' visa for a few years based on my marriage to Thai wife. I am also director of a Thai Company and have a work permit. However I have recently separated from my wife and will need to change my visa to a non immigrant 'B' based on the fact that i am a director of a company here and no longer living with my wife.

Does anyone know if I can change the visa here in Thailand at a Thai immigration office (My local immigration office is Chiang Mai) if so, is this a straight forward process and any idea what documentation i need to bring?

I have spoken to a lawyer quickly on the phone and was advised that it is 'easier' to leave Thailand and apply at a Thai embassy or consulate abroad (i was recommended Penang or KL) however if i can change in Thailand i do not see how a 3-4 day visa trip with flights and hotels as well as ensuring i have all documents required will be easier than changing the visa here in Thailand (If i can do this)

Any advice or information will be really appreciated!

It it is near time to extend again and you are not going to be divorced prior to then you could just extend for reason of employment instead of Thai wife. But if out of cycle believe you should cancel current extension on basis of not living as husband and wife anymore and obtain a new non immigrant B visa for entry and then extend for employment.