Thanks for your reply. My immigration office is in Bangkok and the extension and transfer was done at the same time (not sure if that makes any difference).
Anyway, right now I am still on the window for mail report as it's around 17 days until my due date. My question was only to know if there's going to be any problem to do so after changing passport, as I would like if possible to avoid the trip to the offices.
Last time I didn't get anything for two months... Sent them a mail and they said they'll pass the info to the counter. Got my reply on the mailbox a week later, with some handicraft done to my return envelope (maybe it was too small). Weird stuff.
Thank you, I actually used to get notices of "don't be late next time" when I sent it perfectly on time, so it's not like they are too reliable. I will try and see, last time I mailed it in it took forever for them to send it back too