Hosted Engine is basically a centos stream 8 running the engine. Never tried the conversion but it should be possible.

I suggest asking the Rocky Linux community how to do that.

oVirt node conversion requires either building a node update based on Rocky Linux or a full reinstall of the host with Rocky Linux.

I would suggest going with the full reinstall but you're welcome to take https://github.com/oVirt/ovirt-node-ng-image/issues/46 if you want to try building node based on Rocky Linux.