[Major version upgrade] pg_upgrade fails despite passing check mode

While upgrading from Postgres 11 to 13, the pg_upgrade passed the check mode

postgres(at)dxxxpgs03:/Dxxxx1/datadg/data13$ /usr/pgsql-13/bin/pg_upgrade -k
--check --new-datadir='/Dxxxx1/datadg/data13'
--old-datadir='/Dxxxx1/datadg/data' --new-bind$
r='/usr/pgsql-13/bin' --old-bindir='/usr/pgsql-11/bin'
Performing Consistency Checks
-----------------------------
Checking cluster versions ok
Checking database user is the install user ok
Checking database connection settings ok
Checking for prepared transactions ok
Checking for system-defined composite types in user tables ok
Checking for reg* data types in user tables ok
Checking for contrib/isn with bigint-passing mismatch ok
Checking for tables WITH OIDS ok
Checking for invalid "sql_identifier" user columns ok
Checking for presence of required libraries ok
Checking database user is the install user ok
Checking for prepared transactions ok
Checking for new cluster tablespace directories ok

*Clusters are compatible*

But then while *performing the actual upgrade, it failed* saying the "*owner
of a FDW should be a super user*"

So I started the old Postgres 11 again, changed the owner of that FDW to a
super user.

Now when I try to run the upgrade (even in check mode) again, it says "*new
cluster tablespace directory already exists*"

Is it safe to delete the new tablespace directories?

What are my options to safely start/upgrade the DB without any chances of
data corruption?

Thanks

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