I have been here on a retirement visa for 18 years without a problem, but I am considering changing to a marriage visa to free up some of the capital held on deposit. I have been married (to the same Thai lady) for the same length of time. Is this a good idea. is it difficult and is it worth doing?

It is not difficult to do (done it 9 times already). Just a few more copies of supporting documents, some photos and your wife has to be with you to a statement . Then you will have a 30 day under consideration period before you go back to get the extension stamp.

You will need 400k baht in the bank for 2 months or proof of 40k baht income.

the difference financially   is obvious    the money in bank is probably best at the moment   depends on your countries exchange (fluctuating) rates   as for simplicity   a little more involved  ie different/more paper work   but certainly  doable with patience  

You are aware the money only needs to be in account for 3 months of the year I suspect so if you do not have the funds to continue retirement option (remember you can also use pension combined with bank deposit to lower the deposit amount) then it is a viable option and as said not that difficult - just a few more papers each time and wife with you and a return in 30 days (if retired that should not be a real issue).  Just be sure to not show enough funds for retirement or they may want to use that method as that makes it a local decision and easier for them.