I am an Australian intending to move to Bangkok on a retirement visa. If I subsequently get offered employment with accompanying work permit is it possible for me to change to a work permit? Would that jeopardise future applications for a retirement visa?

In order to apply for the work permit you would probably have to leave the country to obtain a different entry than one from a OA visa or to cancel you extension of stay based upon retirement.

Most work permit offices will not accept a applicant if you are here for retirement.

After the work permit application is accepted you would then need to get a non-b visa for it to be issued.