If you get a new iPhone and restore it from an iCloud backup, your custom ringtones will not be restored, and all your alarms and ringtones will be set to the default Marimba. However, if you sync your new phone with the same iTunes library as your old phone, and the ringtones are restored, all your alarms and phone rings that had custom ringtones assigned will revert to the custom tones automatically.
Thanks Zev Eisenberg