You can amend a return to change from married filling separate to married filing joint..... but not from married filing joint to married filing separate unless you do so prior to the original filling deadline without extensions.
So, once you file a joint return you can not change it to a separate return if the filing deadline has already passed.
-follow these link(s) for more information-
How to Change Your Tax Filing Status - TurboTax Tax Tips ...
How do I switch from filing jointly to filing sepa...
