Is there any way to get one of these little cards without leaving, then coming back and being subject to all the quarantine regulations in place now?  Due to an immigration officer's error at BKK airport in the past I had to get a replacement.  I got a morning flight to Kuala Lumpur and back to BKK, got my new TM6 and went on other formalities from there.  I have a TM6 now which is valid in my UK passport but I would like to get one for my new Canadian passport.  Any way of getting one w/o exiting the country?  Thanks for any ideas or comments.

If you lose your departure card you can get a new one from immigration.  You may need a police report.  But you don't need to go flying around just for that.  What purpose would be served by having another departure card in your Canadian passport?  You only arrived once.

The only way to change passports that are not from the same country is to leave and reenter the country with the other passport.

You can't use your Canadian passport for anything immigration related if you didn't use it to enter the country. So of which use would the TM6 be?

Thanks for the comments.  The short story is that I was married with my Canadian passport (the amphur insisted) and I would just like to tidy things up.  All legal - no hanky panky in any way.  I would take a short flight out and back but not with the pandemic issues complicating things.  My current status is fine and I have another 9 years b4 my UK passport expires.  Thanks again.

Will you be entitled to receive old age pensions from both countries? Just asking

Yes, I get a UK, Canadian and French pension too.  All small but they add up and help to pay the bills.  I keep getting demands from the UK for a 'proof of life' certificate which is a pain in the bottom as it must be signed by an official in English.  One lawyer wanted 30,000 Bt but normally it is 500 or 1000 just to verify that they have seen me standing upright breathing.