Question: will the migration script update the old bugzilla bugs with a "forward pointer" giving the new TNNNNN task? E.g. a comment from Phabricator_migration_user "Converted to Phabricator as Task 28179 https://phabricator.wikimedia.org/T28179"
I think it's useful to have forward as well as backward links between the new and old systems, especially in case something goes wrong, however unlikely :)
Workaround: When you're viewing https://old-bugzilla.wikimedia.org/show_bug.cgi?id=12345 , just remove "old-" from the URL and you'll redirect to the converted task in Phabricator.