Domain migration from FRS to DFS

Hi,

We currently have 4 DCs (2 in each site), 3 of them are 2008 R2 & the 4th is 2016.

We'd like to gradually faze the 2008 R2 out.

We've set up a 2019 server, and upon trying to promote it to be a DC it demanded that we migrate our domain to DFS (screenshot at the bottom).

As I understand the 2008 R2 DCs are capable of that.

I've never done something like this before, and this domain includes an Exchange 2016 server on it (not a DC..).

Is there a step by step procedure I could follow?
Is it dangerous?
What should I do to minimize the risk? (specific type of backups perhaps?)
What about if things go wrong? What might go wrong? How can I prepare & what should I try if something does go wrong?

Thanks in advance!