I haven't seen it mentioned in this post yet, but when you plan to enter visa exempt, you will need proof of onward travel, e.g. a return ticket. This isn't for Thai immigration, it is an airline policy, to protect themselves.
I used the service last month, and it was all ok. Unfortunately immigration have a sign saying "reentry visa" when they really mean reentry permit. The TM8 info on the counter clarified this.
Many years ago I made a return trip from Bangkok to Hong Kong, while on a visa waiver entry. In anticipation of being asked in Hong Kong on the way back, I had bought a train ticket to Malaysia before leaving Thailand. It all worked out exactly as expected, no problem when asked for onward journey proof.
If I were on a short trip here (I am not), I might feel it would be ok to simply leave the country before the end of the 30 days that I should have been stamped in for. After all, travellers should know the immigration requirements while planning their travel. What are your thoughts on this?
I changed my extension type last year from marriage (basis of my original non o visa) to retirement. This was at Samut Prakan IO. I asked at the time if it would be possible later to change the reason for extension to marriage, and they said yes.
It is worth noting that they will cancel your previous extension when you change the reason, and reset the extension date to the date of application. My original extension expired in November and after changing it now expires in September, so don't go too early to make the new application.
Next time I will submit as early as I can, 15 days before. This should give them a chance to process it. Going in person isn't a problem, and you have up to 7 days after the due date to do so. I wonder if it was a coincidence them rejecting after they had approved, or whether it was necessary in their system. As they don't give a reason and only tell you to go to the IO in the rejection email, I do not know.