I am about to do the same. The IO in Korat gave me the form to fill out and asked me to com around 20 days before end of my 60 days visa exemption.
I only need to get an pension income proof from my embassy in Bkk (You might be among those needing 800k in the bank), fill out the papers, have a rental contract, copy of green book and ID card from landlord, 2 pictures and payment for the non-immigrant-O. I think that was it. Had this visa for 10 years but lost it during the Covid <deleted>.