Please note that if you extend a non-immigrant 'o' stay, and change the reason (e.g. marriage to retirement or vice versa), then the date will be reset to the date you submit. This is worth noting as it is possible to submit up to 45 days before expiry, meaning up to 45 days could be lost due to the process of cancelling the original extension due to change of reason.

(At Samut Prakan immigration)