upgrade RH(E)V manager 3.6 to 4.0

I'm testing the upgrade from RHEV 3.6 to RHV 4.0
Since I have to do a complete install from RHEL (7 in stead of 6) I could use a new LUN, do the install/upgrade (RHEL/RHEV-M), test and when there is some issue I just connect my old LUN and continue to use the 3.6 RHEV-M. I will do this while there is no (not much) activity on my environment and I will not upgrade any hypervisor.
May I expect this to work and what action may I definitely not do?
To put it otherwise, following the update procedure, what is the point of no return?