If you add music, what stays grey? and if you want to go back to a unjailbroken device you should "Restore from backup" on iTunes to get your phone back to iOS 8.0.2
Yes, it will to an extent. It will ask to update. Then once you click it, it will tell you the version it's installing. You cannot change the version, you are right but you can remove the jailbreak by installing the new update.
If you add music, what stays grey? and if you want to go back to a unjailbroken device you should "Restore from backup" on iTunes to get your phone back to iOS 8.0.2