@Tod ********
answered same question this morning....re CW Immigration in Bangkok.
However Jomtien DOES REQUIRE return with your updated bankbook in 90 days.
"When you were sitting at the desk of the I/O getting your extension they had you sign several papers (acknowledging overstay penalties, agreeing to contact them if the reason you got your extension changed, AND agreeing to the terms and conditions of your extension).
Chaengwattana does not hand out a paper stating you come back 3 months after you get your extension, they will check the bank balance when you get your next extension to make sure you kept the balance at 800K baht for 3 months after your extension was granted and that the balance never went below 400K for the rest of the year."