Can my husband switch from an OA retirement visa to a paid research visa in Thailand without future implications?

Nope no issue at all switching back to either an OA (which he'd get in his country) or a year extension based on retirement (which he'd get from the immigration office) once he's done with the research visa/extension

but

I am doubtful he can go from an OA entry stamp OR a year extension to a research for thai uni visa/extension inside the country. 😕

However that is between him and whatever uni he's getting the visa from. 🙂