I asked about this here a few years ago and have since migrated two setups successfully. This is unsupported/unofficial, but ... it receives the "works on my machine" certification.
The basic steps are:
- Install 4.2 and 4.3 side by side
- Stop the services for both
- systemctl stop service.tvheadend42
- systemctl stop service.tvheadend43
- Make a backup of both paths
- cp -r /storage/.kodi/userdata/addon_data/service.tvheadend42 /storage/.kodi/userdata/addon_data/service.tvheadend42.BACKUP
- cp -r /storage/.kodi/userdata/addon_data/service.tvheadend43 /storage/.kodi/userdata/addon_data/service.tvheadend43.BACKUP
- Copy the contents of tvheadend42 to tvheadend43
- cp -r /storage/.kodi/userdata/addon_data/service.tvheadend42/* /storage/.kodi/userdata/addon_data/service.tvheadend43/
- Start the 4.3 service
- systemctl start service.tvheadend43
- Log into the 4.3 service and, if necessary, go through the initial setup experience. The service should pick up everything you had configured in 4.2 and just run, but you should double check everything just to be sure. (I don't remember if I did the initial setup before copying everything over or after, but I do remember you have to go through it at least once after you install 4.3)
Disclaimer: It's been 2+ years since I did my last migration, so if I forgot something please add to the thread for the benefit of others who are in the same boat.