I have been in Thailand for a few years, on a retirement visa, with a work permit when the variable Thai policy permitted.

I am thinking of marrying a Thai national, so I presume my reason to stay can change from retirement to marriage. What would I have to do to get a work permit once I am married since I would like to do some part-time work?

Cheers in advance for any information.

You first have to find employment that allows a work permit to be issued and that may not be easy, especially for part time work. Then would would have to change your reason for extension of stay to Thai wife from retirement so that the work permit can be issued.