Update to my own query and a heads-up for others - went to CW, got told by 3 IOs separately (each out of earshot and out of sight of the others) that they wouldn't transfer my current visa extension stamp from my old passport to my new passport. Told me I'd have to apply instead for a second visa extension (the one they reject but you're given 7 days to leave the Kingdom and for which you still have to cough up 1900 baht). Current extension doesn't expire until tomorrow and said to them I was exiting Thailand then, but they weren't having any of it. So left with the current extension stamp still in my old passport. New year, new racket so it seems.