is there any disadvantage / problem, if the Hyper-v role is added afterwards on an active physical Windows server (for example fileserver/dhcp server...) ? It would be better to install it before going productive, for sure. But is there any real problem if it is done afterwards? The server needs to be rebooted afterwards, which is mandatory.
Hyper-V is a type 1 hypervisor, so I assume there is some major change underneath, wenn done.
best regards!
No, there are no drawbacks (unless you have a very specific and very rare scenario like f.e. your OS / application needs an access to some proprietary hardware etc). These days all of the "generic" workload should be virtualized, there's no sense in going bare metal. Things like backup, DR and mgmt are simplified greatly.
Well, I would never put anything else than Hyper-V on a Hyper-V host. Technically nothing should break but it's an extremely bad practice to use your Hyper-V host for other services. You should aim to have the services moved to one or more virtual machines on the hyper-v once in place. Imagine that a user gets hit by ransomware of a nasty kind, which decrypts the passwords on the physical machine, thus being able to encrypt the entire machine and not "just" a virtual fileserver. Also, don't domainjoin your hyper-V to a domain hosted by a domain controller that is a VM on the hyper-v itself. Both for the same reason as above but also to prevent you from locking yourself out completely.