About to do this on my Minisform MS-01, will update this post if successful.
First: doing a complete backup to my Proxmox Backup Server. If you’re not in on this, it works great. Mine is a cheap simple N100 GMK mini hooked up to a 4 drive RAID. It turns on twice a month and all my Proxmox nodes (across 4 machines) get backed up to it. Stays on for the day, then goes back to sleep at night, setting an automatic wakeup for 15 days in the future. Works like a dream.
UPDATE: works, fairly easy.
Instructions for PVE here: https://pve.proxmox.com/wiki/Upgrade_from_8_to_9
PBS here: https://pbs.proxmox.com/wiki/Upgrade_from_3_to_4
Open the shell on the node you want to upgrade, and run pve8to9 --full. I removed 1 package and added the non-free-firmware repository to install the intel microcode, both optional, but I'm a sucker for all green when running a preflight test.
Pay attention to the parts about upgrading your repositories (they provide all the copypasta you need) and about changes to packages. In most cases, just default power through and you'll be fine.
The only gotcha I ran into was I needed to add Enabled: false to etc/apt/sources.list.d/pve-enterprise.sources as I don't use it (I use the community edition, not paid). It didn't impede the upgrade, but future apt update will fail without the change.