The line (20190829) in /usr/ports/UPDATING instructing how to convert database to the newest version fails when databases to convert use the postgis extension:
su -l postgres -c "pg_upgrade -b /tmp/pg-upgrade/usr/local/bin/ -d /usr/local/pgsql/data/ -B /us r/local/bin/ -D /var/db/postgres/data11/ -U pgsql "
It is a version incompatibility problem of databases/postgis* (new version is 2.5, previous version 2.4):
"could not access file "$libdir/postgis-2.4": No such file or directory"
1. to create a simlink (no effect)
2. to install the previous version of postgis in /tmp/pg-upgrade (to do something similar as what is proposed in UPDATING)
ERROR: incompatible library "/tmp/pg-upgrade/usr/local/lib/postgresql/postgis-2.4.so": version mismatch
Sep 17 21:39:40 <local0.warn> * postgres: [3-2] DETAIL: Server is version 9.5, library is version 9.4.
Sorry for this late answer. From your error message, it seems to me that you simply installed the wrong version of the library?