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
froma  newer  release             of  PostgreSQL, or when your template1 database has             become corrupted by
somesystem  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.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver              PGP key from public servers; key
ID32B8FAA1                ========================================    "If the Son therefore shall make you free, ye
shallbe      free indeed."         John 8:36