Yipes. Not good. long story short, change your DHCP scope options to include a new DNS server of different servers first. Make sure there are NO WINS entries if you dont use WINS. Let that cook for a few days so everyone gets the new settings. Go through your static IP machines and make sure they are changed too. Then demote and change your DNS server entries between servers.
If something messes up, you can make the machine name a domain admin and it will fix your problems. You can then take those machine names out after a few days of propagation. Just for your notes, any server software installation that is Microsoft based (exchange, crm, etc) or anything that uses authentication, will fail if you dont do the above mentioned.