My daughter and I plan to remain in Bangkok for 1 1/2 years after my husband relocates for business to another country.  (We are all Brisih citzens,

my minor daughter attends an international school).  I am aware of the financial requirments to obtain an extension based on guardianship to a minor

and documentation required from the school

My question has to do with timing in the  change to our status.  Our current extensions based on being dependents to work permit holder are valid

though March 2018 although I know this will be cancelled as soon as my husband leaves his current employment at the end of December 2017.

Your husband will need to cancel his extension of stay using a termination letter on or before (up to about 7 days max) the date on the letter.

When his extension is canceled is when you and your daughter would have to apply for new extensions.

You daughter will need paperwork from the school to apply for an extension based upon attending school.

You would need to prove you are her mother by showing her birth certificate to apply for an extension based upon being her parent (not her guardian). That is when you will need proof you have 500k baht in a Thai bank in your name for 30 days or possibley 90 days if they did decide it is not your first extension application.