I am going through the process at the moment. I hired an agent who can help me. I am currently on a Tourist visa with the 30 day extension. The first 3 months of retirement visa has to be converted to a Non B visa which means you have to be available to go to immigration and then you will have to go back after 90 days. It's a little complicated if you are entering a Tourist visa.
You can request for a medical visa if you are scheduled for surgery by your doctor. You will need to read up on the requirements if it matches your needs. That will give you 90 days to stay. If you need further details on it Bumrungrad Hospital has their own visa place. They might be able to give you more guidance on this