Yes, the server will need to be demoted from the existing domain by running dcpromo, then dcpromo'ed back into the new domain once it is moved and connected to the new domain.

Before DCPROMO'ing the old server out of the domain, another DC needs to take on all the responsibilities the server currently has if you want the domain to still remain, and this work needs to be performed BEFORE dcpromo'ing the server away.

To correctly remove the Domain Controller, you will need to:

Ensure DHCP, DNS and Global Catalog roles are moved across to one or more of your new servers. If you are using AD-integrated DNS (which you should) this is as simple as installing the DNS server on the new server (DNS information will replicate with AD) and the DHCP configuration simply needs to be copied from one server or the other. Make sure the DNS server addresses in DHCP AND any statically assigned devices point to one new server for primary, and the other new server for secondary DNS if you install DNS onto it.
For DHCP if you spread it around multiple servers, it is as simple as setting IP scopes on the correct subnet which DON'T overlap (otherwise both servers will give out the same addresses and you will have IP conflicts)

Make sure all the FSMO roles are removed from the first server and transferred to one of the other servers. If you are a single-domain environment, there are no performance or functionality gains from spreading FSMO roles around between servers. They COULD be spread around - it is up to you, but it is recommended they are consolidated onto one server. FSMO transfer guide here: http://support.microsoft.com/kb/324801 and you may like this one: http://www.petri.co.il/transferring_fsmo_roles.htm.
Note you can test if the current server has any FSMO roles by running (at a command prompt)
netdom query fsmo
and examining the output of server FQDNs.
If you see anything about SEIZING FSMO roles, DO NOT undertake this procedure, in this case you can TRANSFER the roles which is a less painful procedure.

It seems you have already migrated DNS and GC roles across to the new server, so that's not a problem. Make sure the new servers are correctly replicating information with Active Directory before proceeding.

As a test, shut down the existing DC and make sure all Active Directory and client workstations are functioning properly. If they are, you should be safe to go ahead and run DCPROMO to demote the DC to member server.

Make sure then replicate any data and shares off the server if you intend on completely removing it from your network.

-tigermatt