Bonus Users - How to Use

Ok, I do not wish to look a gift horse in the mouth, but I am slightly confused about the five bonus users from referral.

If I upgrade my account, do these five upgrade too or do they remain basically free user accounts rather than upgraded ones?

Also what happens to my pre-existing Zoho email accounts? Do they remain the same as they are, or do I have to upgrade all of them too? If so, can the five bonus accounts be used for them?