you could go from a visa-exempt entry or on the last 60 days you will receive when doing a last border run under the still valid METV, to the 90-days Non-imm-O retirement visa, at a Thai Immigration, by the procedure "change of visa type", if you already have deposited 800.000.- THB in your Thai bank account and can prove the money came from abroad. However you must wait for the "validity period" of your METV to be finished, before you can apply for the "change". One cannot get a new visa as long as one still is on a valid visa. The earliest day to apply, is the day after the expiry of the METV, i.e. after the last border run. Check here for the requirement, click onto number 9 is for the change to the "90 days Non-Imm-O retirement visa" and from this visa finally to the "1-year extension of stay permit"
**********************************************