Can I use seasoned money immediately after the marriage extension approval in Thailand?

Believe it or not there is nothing in the immigration rules about marriage extensions that say anything other than the 400K needs to be in the bank for 2 months before you apply for an extension.

Nothing says you need to keep it in during the under consideration period <- (although some offices were telling people) 😕

You're fine once you get the extension to do what ever you want with the money there is no minimum balance or anything like that