Can I switch to a marriage visa extension while currently on a Non-O Retirement visa using monthly income deposits?

Be aware you have to be legally married for it to count for anything and there are a couple of hurdles to climb such as proving you are free to marry. Suggest you start looking into all that now if you wish to get legally married in July.

the reason I ask IS

If you used banked money method last year to get your yearly extension the terms you have to follow are; have 800K baht in the account for 2 months before you applied, you kept the 800K baht in the account for 3 months after the extension was granted and don't let the balance go below 400K baht the rest of the year.

Changing the reason for your extension isn't going to waive any of those previous requirements because you're here now on the extension based on retirement and have to follow those terms up until the day you change the reason.

I'd say seeing as you can't have the bank account go below 400K baht to meet the terms of THIS extension, just use the 400K baht banked money method for your first marriage extension, keep bringing in the 40K baht a month and as long as you do that for the previous 12 months you can use them to meet the proof of funds for monthly income method next year no problem.

okay, you're switching extensions right? <- As in last year you got a yearly extension based on being over 50 (retirement which you used banked money method to meet the requirements) and this year you're wanting to apply for a yearly extension for a different reason (based on marriage to a thai) CORRECT?