Lots of questions about in place upgrades. Keep in mind that we invested time and resources in Windows Server 2019, and even more in Windows Server 2022, to make in place upgrades "just work." The Windows Server team support in place upgrades (up to N+2), but in place upgrades is more than the operating system. Ask yourself two questions:

What is the app you are upgrading?

Is this an inbox app (role) like file server, IIS, print? It should just work. (NOTE: Domain controllers are a special case. For DCs, install new DC, sync with old, promote new and demote old.)

Is this an app installed on the OS? SQL Server? Exchange? SharePoint? 3rd Party App? Consult their documentation. Some apps may support it, some may not. The support decision is up to the app owner. For example, if you have some Oracle app running on Windows Server and Oracle states they don't support in place upgrades, that's their support decision.

What is the underlying platform?

Is this app running in a virtual machine or on hardware?

If the app is running in a VM, things are easier because the virtualized hardware will likely look the same. For Hyper-V, the guest drivers are automatically updated, so it's EASY. For third party virtualization, see them and don't forget to update the guest drivers. A common support issue is that folks are running ancient 3rd party VM guest drivers.

If the app is running on hardware, proceed with caution especially in the N+2 scenario. For example, imagine you have hardware originally running Windows Server 2012 R2 and now you want to in place upgrade to Windows Server 2019. That's a big span.

1. Is the new hardware certified for the new OS?
2. Are there drivers for the new OS? Be sure to look at I/O. Storage controllers, Network Adapters.

Backups

Finally, always have backup before performing in place upgrades. If you're virtualized this is easy. If you're on hardware make sure you perform an appropriate backup.

My experience is that Small Business Server 2011 and earlier that came bundled with Exchange and or Sharepoint can be a PITA to upgrade. Best practices dictate creating new VM with modern OS and migrate roles and features. Or for smaller organizations, just create a new Domain Structure, and either migrate accounts via domain trust relationship or manually recreate the OUs and Users, remove workstations from old domain and join to new domain.