@Darin *******
you can apply for a "change of visa type" from a tourist visa or a visa-exempt entry, to the "90-days Non-Imm-O retirement visa" on Immigration.
You must be able to prove
EITHER
an income of a minimum of 65,000.- THB monthly,
OR
have a minimum of 800,000.- THB sitting in your Thai bank account on the day you apply.
Also there must be a minimum of 15 work days left on your stay permit stamp.
The 90-days Non-Imm-O visa will be issued "under consideration" for 14 days. As soon as 30 days are left of the 90 days and the money has been seasoning in your account for two months, you can apply for the "1-year Extension of Stay Permit" based on being over 50 years/retirement