Hello. I have had great success with migrations of all types using a program called MigrationWiz from BitTitan. They allow you to import bulk users and passwords from different types of files (csv etc..) and it can be used for almost any mail platform migration. The only problem is, it can get a bit costly. They charge on a per-mailbox basis. We always purchase from a reseller which is a few bucks cheaper, but on a large scale like you are planning on doing, I'm not sure if it is worth it to you. You may want to go to BitTitan directly and ask for a discount since you have 600+ users.