Removing Active Directory (AD) from a Domain Controller (DC) in a single domain environment where there are other replica DCs is pretty easy. Be sure that the remaining DC is flagged as a Global Catalog (GC) server (visible in "Active Directory Sites and Services" in the properties for the "NTDS Settings" under the DC object). If it isn't, flag it as such, because you need at least one GC at all times.
Just be sure that the machine has as its first specified DNS server the IP address of another computer hosting a copy of the AD domain's DNS and run
dcpromo.exe, taking the defaults (be sure NOT to specify that this is the last DC in the domain).
When you say "...we did the AD, WINS, File Server, DNS and DHCP roles. We are going to keep it as a file server and demote everything else.", however, you make me think that you're also looking at moving DHCP, DNS, and WINS off this machine. Removing the functionality those services provide involves some planning.
You can use the
netsh dhcp server dump > dump-filename command to dump the DHCP server configuration into a text file (named "dump-filename", in this example) which can be imported onto a new DHCP server using
netsh -f dump-filename.
You'll want to be sure that your DHCP scopes don't reference the DNS or WINS services provided by the machine you'll be removing the services from. You'll also want to be sure that machines with static IP configurations also don't reference these services.
You'll want to remove the WINS server from any replication configuration on other WINS servers.
Having said all this, you really should have at least two DCs at all times. Don't migrate to a configuration with a single DC.