@Pe**
, That is a good question.
I had a friend of mine that needed to change his expiration date because he could no longer be in thailand during the time it expired.
It was hard for the the immigration officers to understand
A ) he needed to change the expiration date
B ) he didn't want a full year's extension this time
Finally they got it and we applied for an 8 month extension instead of the year (same 1900baht).
Honestly it was like trying to push a rope :O
So yes, it can be done, but . ..