Thanks for the reply, so the upgrade procedure would be something like this?
add this in nano /etc/apt/sources.list
deb http://download.proxmox.com/debian/pve stretch pve-no-subscription
then run
apt update
apt upgrade
apt clean && apt autoclean
then reboot
Stoiko's reply contains the link to the procedure I followed. Sorry I didn't add it in my former response, I supposed you had already read it and was unsure about the "go or not go".
Depending on how mission critical is your cluster and your experience level, you may feel uncomfortable with the upgrading. In such case I'm not a Proxmox commercial but you can always get the support level you need with one of the available paid subscriptions, which btw unlocks access to the enterprise repo too ;-)
In my case the upgrading has proven to be worth the effort - by far.
Regards,
Marco