After having completed an upgrade from 1.6 to 2.2 via launching new instances, adding them to the ReplicaSet and letting them update, and then removing the old instances, do I still need to reindex to get more performance? Or did the replication (versus upgrading the Mongo version on the same instance with the old data files) handle the newer and better index style already?

If I do have to re-index, can it be done without downtime?