So I had my PowerEdge R420 boot from a USB with ESXi 6.7 and only one hard drive (1TB) (first build). Now when I tried to install VCSA 6.5 I couldn't get it to work and read online that VCSA 6.5 is not compatible with 6.7 ESXi. Now I installed a 6.5 ISO on a USB and tried to boot from that. When I try to install it on my hard drive it gives me an error that I can't over ride the current Hard Drive. My question is how do I delete the partition from the Hard Drive with no ESXi 6.7 installed on the bootable USB.
vCenter must be always in the same or superior version than the ESXi hosts, so if you updated a new ESXi host to a newer version than your vCenter you must be checked this before.
About the ESXi host, were you able to return to the previous version of the ESXi?
Per default the hypervisor saves the latest previous build so it can be reverted.
Here is more information about how to do it: VMware Knowledge Base