@Ian ****************
the information Brandon, Chris and John have given you is correct. You can do a border bounce without getting a re entry permit. Then you enter Thailand receiving a visa exempt stamp on arrival which gives you 30 days. This you can extend with another 30 days at an immigration bureau in the country. An agent can do it for you if you don't want to travel to the immigration office.
Within this time you'll need to get all your paperwork sorted as you'll have to apply for a non-o 90 days with still at least 15 days left. After those 90 days you can apply for the one year retirement visa.
Using an agent is highly recommended, immigration often does not like having to put too much time into getting you to understand what papers the Thai government requires as it's a lot. If you have any specific questions or need more recommendations feel free to message me. My husband works closely with immigration and can offer you complete clarity on the whole process.