@Kenny *******
I meant to pull all of Bo's questions and answers out because they do not have anything to do with your question.
As people said correctly you need to BOTH meet the requirements for your current extension based on retirement (keeping the minimum 400K baht in the bank until next year) AND you need to meet the requirement for the next extension based on marriage (keeping 400K baht in the bank for 2 months before you apply for your next extension).
I would tell you the same thing I told Bo, GO to your immgiration office well before your current extension will expire, tell them you're going to "switch the reason for your next extension" and get the list of documents you need to do it.
That way you're not waiting until the last minute before your current extension expires and then running around like a head with your chicken cut off tryin to round up the documentation.
Good Luck