Just like Strahil, I would expect this to work just fine. RAM differences are actually the smallest concern, unless you run out of it in the mean-time. And there you may want to be careful and perhaps move VMs around manually with such a small HCI setup. oVirt will properly optimize the VMs and the hosts to fit, but I don't know what it will do when there simply isn't enough RAM to run the live VMs. Under the best of circumstances it should refuse to shut down the node you want to upgrade. Under less advantagious circumstances some VMs might get paused, or shut down (or killed?). I'd be interested to hear your experiences, somewhat less inclinded to try myself ;-) I'd play it safe and reduce the number of running VMs to relieve the RAM pressure. I assume there is a reason you want more RAM but the only way to get there is to reduce it first and that doesn't imply "unnoticeable".