I want to upgrade the Server 2008 domain controller to Server 2012 R2. (Reason we don't go for Server 2016 or 2019, we have only server 2012 r2 licneses)
I have already installed a Server 2012 R2 server and added to the domain.
If I don't run forestprep and domainprep, would that be a problem?
On 2012 I believe it does this when you install the AD role on the server.
Realistically, going to 2016 would be preferable purely because 2019 has no additional functionality over the 2016 DFL and FFL.
Also, 2012 doesn't fully support windows 10, it's a bit of a pain really.