So this is for an Extension of Stay based on a Non-O (retirement)? If not and it is for an Extension of Stay based on a Non-O (marriage or Thai child) you just need to be at 400k 2 months before you apply.
It's basically the same things required for your extension but the funds do not need to be seasoned (unless you apply at Jomtien) and there is no financial check over the past year