My home office lab server was running Proxmox 4.x which was clean installed ~late-2016 onto suitable spare hardware (quadcore AMD, 8gigs ram, pair of 1Tb SATA Drives) via standard 'custom manual' process (ie, Install minimal debian 8 first and manually config software raid, then install proxmox after via apt source edits). This is of course quite painless and smooth.
In the past, upgrade of proxmox 'in place' had been a bit of an adventure so I was curious how it would go on more recent flavour.
This was done and the net result is that - it is painless, even for a box running atypical config (ie, software raid) under the hood.
General steps followed were simply as per the docs at Proxmox, as per https://pve.proxmox.com/wiki/Upgrade_from_4.x_to_5.0
and it really was smooth sailing, entirely painless. Clearly it takes a little while to download all the update packages - approx 800 megs - and then a while to install everything. But it really did come down to,
Generally speaking, Proxmox 4 to Proxmox5 is more about the debian version change under the hood - the core cluster management toolkit is more or less the same (apparently) to the point that proxmox docs suggest you can tolerate a 'hybrid upgrade' on a cluster, ie,