Since 1 October 2006 new rules have been put in place that affect applications for extensions on Non Immigrant “O” visas .
However, these changes have a grandfather clause included. The grandfather clause “is an exception which allows something pre-existing to remain as it is, despite a change to the contrary in the rules applied to newer situations.”
In laymans terms this means that if you were previously granted an extension before October 1st 2006, then you will continue to be able to obtain an extension under the same rules as you did previous to October 1st 2006
However, if you make a change to the reason for extension (ie change from based on marriage to retirement ) then the grandfather clause will no longer apply with any subsequent application, even if you revert to the original reason.
The grandfather clause now applies only to retirement and investment visa extensions. Extensions based on marriage to a Thai national are no longer grandfathered.