What do I need for my first 90-day report in Bangkok after converting to a retirement visa?

Always do early, I'd never go after the stamped date, sometimes something goes wrong and you need those extra days to fix things.

Save yourself a trip and mail it in. Or try online first and see if they reject. They normally respond in hours. Many people including myself have successfully did first 90 day online. It doesn't hurt to try or but they accept mail in regardless. Just go to post office. You have options. I know in issan it's to do first 90 day online. Bangkok is different but they online first. The most that can happens is they don't accept it. There is a video from immigration and it does not state it's mandatory to report online. My wife showed me the video. More than willing to show proof of mine

you go to the main immigration office at Chaengwattana and you want section A (the 90 day report room)

𝗜𝘀 𝘁𝗵𝗲 𝟭𝟰 𝗱𝗮𝘆 𝗽𝗿𝗶𝗼𝗿 𝗮𝗻𝗱 𝟳 𝗱𝗮𝘆 𝗮𝗳𝘁𝗲𝗿 𝗿𝗲𝗽𝗼𝗿𝘁𝗶𝗻𝗴 𝗱𝗮𝘆 𝗿𝘂𝗹𝗲 𝘀𝘁𝗶𝗹𝗹 𝗶𝗻 𝗲𝗳𝗳𝗲𝗰𝘁?

Yes, you can file a 90 day report in person from 2 weeks before until 1 week after the due date

𝗪𝗵𝗮𝘁 𝗼𝘁𝗵𝗲𝗿 𝗶𝘁𝗲𝗺𝘀 𝗱𝗼 𝗜 𝗻𝗲𝗲𝗱 𝗼𝗿 𝗮𝗿𝗲 𝗿𝗲𝗰𝗼𝗺𝗺𝗲𝗻𝗱𝗲𝗱 𝘁𝗵𝗮𝘁 𝗜 𝗯𝗿𝗶𝗻𝗴 𝘄𝗶𝘁𝗵 𝗺𝗲?

TM30 (which should be in your passport anyway)

𝗜𝘀 𝘁𝗵𝗲𝗿𝗲 𝗮 𝗳𝗼𝗿𝗺 𝗜 𝗰𝗮𝗻 𝗽𝗶𝗰𝗸 𝘂𝗽 𝗮𝘁 𝘁𝗵𝗲 𝗶𝗻𝗳𝗼𝗿𝗺𝗮𝘁𝗶𝗼𝗻 𝗱𝗲𝘀𝗸?

Get the TM47 form at the desk when you go in OR download it from the imimgration website

𝗪𝗵𝗮𝘁 𝗰𝗼𝗽𝗶𝗲𝘀, 𝗶𝗳 𝗮𝗻𝘆, 𝘀𝗵𝗼𝘂𝗹𝗱 𝗜 𝗺𝗮𝗸𝗲 𝗯𝗲𝗳𝗼𝗿𝗲 𝗜 𝗲𝗻𝘁𝗲𝗿 𝘁𝗵𝗲 𝗾𝘂𝗲𝘂𝗲?

None, there are no copies required in Bangkok to do a 90 day report