For your scenario, i'd probably either just bite the bullet and migrate everyone over (maybe giving a permanent discount to the users only using email) OR Create a "hidden" tier not available to the public for these specific users, that way they can be grandfathered onto the old plan and not get screwed by the new pricing.
As for grace period, I usually warn people at a minimum one billing period away. If it's a product that takes a large effort to move off of i'd probably recommend a bit more time as you don't want to force people to keep paying at a price they aren't comfortable with.