I am currently in thailand on a tourist visa. I am also married to my thai wife and I need to change my visa to a marriage one. I am confused my the requirement of the visa.

As I understand it so far, I can go to Laos and apply for a non immigrant O visa, with the relevant paperwork and a bank statement from a thai bank showing 400,000baht for a minimum of 2 months? Is this correct?

I then come back into Thailand on the new visa O, and 90 days later I change this visa to a marriage visa at in immigration office in Thailand. Do I also at the point in changing the visa show again 400000baht? Ie do I have to have 400,000baht in my bank for 5 months now.

You will not need to show the 400k baht in the bank to get the non-o visa based upon marriage in Laos.

You will need the 400k baht to be in the bank for 2 months when you apply for an extension of stay (it is not a visa) based upon marriage at immigration. You can apply for the extension during the last 30 days of the 90 entry you will get from the visa.