[ovirt-users] Re: Upgrade from oVirt engine 3.6 to 4.0 - clean install of OS?

The upgrade guide says you must go 3.6 -> 4.0 -> 4.1 etc https://www.ovirt.org/documentation/upgrade-guide/chap-Upgrading_from_3.6_to... I just want to know if I really, really have to bare metal install of CentOS all over? That seems crazy in a Linux world.