Unfortunately the IO was correct you cannot change from a OA visa to a Non O visa in county. So you must leave Thailand, and either apply for the Non O visa in an embassy/ consulate or you can come back into Thailand on a visa exempt stamp the apply here for the Non O visa.
correct Brandon, I should have said if you qualify it isn’t a hard process. I have 15 friends that have achieved it , and I will be doing it myself next year, after 46 years traveling and living here, and being married to my Thai wife for the last 37 years and having had a work permit for the last 6 years.
Nobody can give you a yes or no answer, except the immigration office who will be standing in front of you, entry will be entirely at their discretion. But be prepared for some questions.