When you say managed switches are you using any VLANs?
One advice change the lease period on your DHCP server from whatever is set to to a very short time at least one week before you start the task other wise you will have to wait until the lease time expires or yourself manually releasing and renewing on every single workstations.
always decide what IP addressing you will be giving to your servers so start at the top (servers first) by doing that your clients will be able to login and get on with thier jobs (may be no internet and mail for a short while), second update your firewall (internal interface then anyrule you set
The DCs need to be approched another way as you need to get on the dns console and update some properties on the SRVs
It is hard to do I’ve done it many times there is no best approach depends on the structure of your network.