Carrier was supposed to transfer the service from the old micro-SIM to the new nano-SIM within four hours. I lodged the application at 12:32 pm today (Saturday). Their active hours on Saturdays are 9am to 6pm so there was plenty of time to do the job.
So I got the scissors ✂ out and cut the existing micro-SIM down to nano-SIM size. It's working, somewhat to my surprise.