Upgrading 3.5 clusters advise

So I have a single Datacenter with 2 clusters. One cluster is on 3.5 and the other cluster is on 3.6 compatibility. There is a seperate VM (non-host) running the Hosted Engine.

Just spun up a new Host on the latest RHEV-Hypervisor for a new Data center which is showing 3.6 compatible.

I guess I was expecting to see a higher compatibility in that new data center.

I understand I will not be able to upgrade the Datacenter with a 3.5 cluster.

reading: 5.5. Upgrading Hosts in a Self-Hosted Engine Environment
https://access.redhat.com/documentation/en-us/red_hat_virtualization/4.0/html-single/self-hosted_engine_guide/index#Upgrading_the_Self-Hosted_Engine

Do I need to upgrade the hosted Engine or the brand new Host?
I imagine I should be able to do this without taking any hosts down and regardless that the one cluster is 3.5.