Windows inplace-upgrade of PI DA and PI AF Server

We have exactly the same question!

The only thing I know so far is that to get to 2022 we will need to go via 2016 or 2019 (https://learn.microsoft.com/en-us/windows-server/get-started/upgrade-overview) and that MS keeps recommending a clean install.

For us thats not an issue as we will be migrating to a new environment (Clean installs) by end of year but our new datacenter provider will only allow us to migrate like to like, so we need to be running the same target OS before.

After posting the above I found a guide from OSIsoft:

https://customers.osisoft.com/s/knowledgearticle?knowledgeArticleUrl=How-to-perform-an-In-Place-Windows-Operating-System-Upgrade-on-the-PI-Data-Archive-machine

It depends on your environment. On-premise, your Windows team will likely not want to do an in-place upgrade. If you're in the cloud, it's easier to do an in-place upgrade as compared to on-premise and I've done them before (not PI-specific) without issue.