I am  in possession of a toursit visa multy entry that expires on 20 September 2018.
I would like to convert this into a retirement visa.
Where can I do that in Bangkok and what forms should I have for it.
I am 54 years old and have more than 800,000 baths on my Bannkok bank.
Sorry for my bad English
Please feedback.
Guido (Lat Phrao) Belgium.

I'll move your question to the correct forum.

You will do everything at the Chaeng Wattana immigration office. Map and info getting there: http://bangkok.immigration.go.th/en/location1.html

First you will need to apply for a change of visa status to obtain a 90 day non immigrant visa (category O) entry. You must have 15 days remaining of the 60 day entry from your visa or the 30 day extension of it.

Requirements for the change is here. https://www.immigration.go.th/content/service_80

Then during the last 45 days days of the 90 day entry from the visa you can apply for a one year extension of stay based upon retirement. The 800k baht will have to be in the bank for 60 days on the date you apply.