Hi clvx,
I’m very new to nextcloud. But in the Administrator manual, you can find everything you need about postgresql database in basically:
As you have an existing installation with mysql database and files, in the data directory, you want to export your mysql
via sql dump and import this into your new postgresql database. Installation hints in the manual above.
Second part are the data directory and third the html and php site.
Keep in mind, you will need postgresql support in php. You find this also in the manual.
I’m installed nextcloud with postgresql database from the ground. I don’t migrade from mysql to postgresql, and it works fine. Hopefully this helps you.
bye.
PS: Also take a look here:
https://docs.nextcloud.com/server/stable/admin_manual/configuration_database/db_conversion.html