I am currently residing in Thailand on a retirement visa with multiple re-entry permit. An opportunity has arisen to start working (legally) in Thailand and I am unsure of the process to convert existing visa to working one. Eg do I have to cancel retirement visa and leave Thailand or can I start work permit/visa process while still here? Will my multiple re-entry permit still be valid - if I need to get new visa?

Thank you in advance.

The majority of work permit offices will not accept a application when a person is on an extension of stay (it not a visa) based upon retirement.

You will likely have to leave the country to get tourist visa or visa exempt entry to apply for a work permit.

Immigration would normally not cancel your extension based upon retirement since a valid reason is needed to do it. You would have to leave the country and ask the officer on entry not to use your re-entry permit.

Then after getting the a work permit application approval letter you would have to get a single entry non-b visa for the work permit to be issued.

No matter how you do it your current re-entry permit would no longer be valid.

I did the same conversion eight months ago. I had to leave Thailand to get my Non-immigrant "B" visa at a Thai consulate. Have your employer prepare all the necessary documents. All you have to do  is make the trip.

If you make the change, you will void your Re-entry Permit.

If you are married get a Single O and convert to extension based on being married, yes, you can get a WP on it. The extension won't be cancelled at the end of the job because it's based on being married. 

An extension based on work finishes the same day as the work.

I've worked on multi entry O visas for at least 10 years and kept each visa at the end of every job, means out/in every 90 days but I felt ok with that.

Thanks for your responses and insight