When I retired from Mahidol, I like you thought I would need to change my visa from Non-B to Non-O. As it ended up, however, when I went to Nakhon Pathom immigration (where Mahidol is located and where university employees were directed for extensions), the immigration officer said not so. She simply wrote a note in my passport underneath the extension that was expiring, issued me a letter, and sent me to Chaengwattana, because I actually live in Bangkok and would henceforth need to do my extensions there. Subsequently, I extended on my Non-B for purposes of retirement at Chaengwattana and have now done so for nine consecutive years.