I did it earlier this year. US Embassy not issuing the letter anymore. I took my old and new passport to the Koh Samui Immigration and they transferred the stamp. Had to fill out a document or two.
For me, when I am logged in, sometimes the system hangs, so I 'X' out of it and then attempt to sign back in again. That is when I get that message. But I don't seem to get the message if I wait more than 15 or so minutes to log back in. Regardless, if I see the message then I will click 'ok' to override.