Please help demote a third DC.

Hello,

I have an old WIndows 2012 R2 DC (old-dc) that used to be the primary domain controller.

I have a couple of newer 2016 DCs (new-dc1 and new-dc2).

I want to demote and turn off the old domain controller.

On the newer DC1, I ran netdom query and it has all the fsmo roles. I also ran replication admin to see that everything was replicating fine.

I went to server manager on the old 2012 R2 DC to demote it.

I left force removal and last domain controller unchecked.

It says this server has dns and GC roles (I think I select proceed?) and next

Then it leaves me with the box remove this dns zone – I can't go next without it. I'm not sure I should. Wouldn't that remove it from the other DCs?