What are the steps to change from a marriage visa to a retirement visa in Thailand?

If you’re on extension of stay, you can apply for or change extension from marriage to retirement. This is normally possible from your second extension of stay.

You just apply before your current extension expires and need to fulfil the general requirement for an extension of stay based on a regular Non O including two months seasoning of 800K baht before you apply, unless you’re embassy offers to give you a affidavit letter.

If you’re really on a one year Non O visa with multiple entries based on marriage, you’ll need to cancel this and apply for a new 90 days Non O based on retirement from scratch.

You're most likely not on a visa, you're on an extension of stay based on being married to a Thai.

When your extension ends and you're applying for your next extension, you can change the reason of your application to being over 50 (retirement) instead.