if you want to use command line to change it , first you need to get a annex B firmware, but i checked tp-link website , it seems this model has no annex B version.
so i think you can't do that by yourself, also Annex A and B have a little difference from the hardware, so it's not a good way to use annex B firmware to upgrade annex A device , maybe it will cause performance down or damage device.

Thanks cake, so the question is: will there ever bee an Annex B firmware for that device? Or is any compatible one available?

Jackie wrote

Thanks cake, so the question is: will there ever bee an Annex B firmware for that device? Or is any compatible one available?

Hi, Jackie. Not sure if there is an Annex B firmware for TD-8840.
I find that TD-W8961NB and TD-W8951NB can support Annex B after searching TP-LINK website.
Hope this could help you.