[ovirt-users] upgrade hosted-engine os ( not hosts )

I am looking for a way to get my hosted-engine running on el7 so I can upgrade to oVirt 4.0. Currently my hosts already run el7, but my hosted-engine is still el6. I read https://www.ovirt.org/documentation/how-to/hosted-engine-host-OS-upgrade/ but this is only about the hosts. I read https://www.ovirt.org/documentation/how-to/hosted-engine/, but it only mentions upgrade of the hosted-engine software, not the OS. I understood I can do a fresh hosted-engine install, and then import my storage domain to the new hosted engine, but: - Do I need to restore my hosted engine database? ( like described here: http://www.ovirt.org/develop/developer-guide/engine/migrate-to-hosted-eng... ) - Can I directly install hosted-engine 4.0 and then import the storage domain? Or should I install same hosted-engine version? - Do I first need another master storage domain or can I directly import my old master storage domain? - When importing the storage domain what is the risk it fails ( I have backups, but it would cost a day to restore all ) - How long would import take? few minutes or hours? ( I want to keep down time as low as possible ). Another option would be upgrade the OS ( with redhat-upgrade-tool ) or is this a path for disaster? I hope someone can tell me how I can smoothly upgrade my hosted-engine up to el7 and run oVirt 4.