Can I replace my non-B visa at Samyan or only at Chaengwattana?

You need to replace your Non-B visa at the immigration office that issued your current extension. If it was issued at Chaengwattana, you must go there for the replacement. Note that the Samyan office is no longer available for this service as it has moved to the One Bangkok Building, and you can only use that office if your visa was originally issued there. Additionally, you can inquire about the possibility of making an online appointment for the service.