Its that time of the year again and i currently have a Retirement extension but with the current rates I do not have the 800k in the bank is it possible to change my Visa to a Marriage Visa without leaving the country.

Yes. Apply for the 12 month extension based on having a Thai Wife.

You don't get a new visa, you just change your reason for an extension from retirement to marriage with a Thai.

You are aware that you also have the possibility of showing a combination of money in the bank and your yearly income (which can be from abroad? If they together total 800,000 you can still get an extension based on retirement, which would be less trouble than based on marriage.