The -k or --link option makes hard links. Therefore, after the upgrade you can safely wipe out the entire old 9.1 data directory. I removed the 8.4 data directory without affecting the 9.2 data. After pg_upgrade is finished, it even generates a shell script called "delete_old_cluster.sh" so you can run it to remove the old data directory along with some 9.2 temp tablespace files.