How do I prove salary requirements for a Marriage Visa after transferring employers in Thailand?

You aren't gonna be doing anything as far as getting a marriage VISA :O all you'd be doing is changing the reason for your extension to one based on employment to one based on marriage

You'd cancel your extension with the current employer; if you're in Bangkok you'd do that at the Cham Churi Square One Stop Service Center Location (that handles BOI people),

Then you'd go to Chaengwattana and apply for brand new extension of stay based on marriage by meeting the proof of funds, (which is either 400K baht banked in a thai bank account in your name only for 2 months before you apply OR proof you have 40K baht a month in income for the previous 12 months before you apply for the extension. :/

There's no way to do meet the proof of funds using monthly income method on a marriage Extension IF you are changing employers at the same time.

you needed to change to a marriage visa extension BEFORE you left your current job so you could use that salary to qualify for the 40K baht a month in income from legally working here for the required time before you apply for the marriage extension.