Question about initdb -t

The man page for initdb says that the -t option will overwrite the template1
database with the up-to-date structure and suggests that this is the way to
upgrade to a newer release:

--template

-t Replace the template1 database in an existing
database system, and don't touch anything else.
This is useful when you need to upgrade your tem­
plate1 database using initdb from a newer release
of PostgreSQL, or when your template1 database has
become corrupted by some system problem. Normally
the contents of template1 remain constant through­
out the life of the database system. You can't
destroy anything by running initdb with the --tem­
plate option.

How does this relate to pg_upgrade? Is it necessary to do a pg_dump? If
not, is there a danger of new oids in template1 overwriting old oids in
user data? (I am not clear whether oids are unique to a database or
to the whole PostgreSQL installation.)

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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