That's all easily moved over. The point is that when the user's password is hashed in the old DB. We aren't going to just copy that over, the user will need to access a link and that link needs to be sent over securely. Email is the best option right now.
why not?
if you use a different hash algo you may just have a flag which recalcutes after first login
Mostly because we won't have access to it.