Yes -- if something goes wrong, you can put the BDC back into place, shut down the newly upgraded PDC and then promote the BDC. However, life is not perfect. Any Windows 2000 machine (server or workstation) that was rebooted post-Windows 2000 upgrade will have altered the way it operates. You will need to take such servers out of the domain and rejoin the NT4 domain.
The NetBIOS name is used in WINS registration and other NetBIOS activities. If the Web server is a Windows 2000 machine, it will register the WEB name with the DNS. The www.acme.com is a public Web site URL, a different type of animal. You manually have to put the www.acme.com in the DNS; it is not registered automatically.
If you change the IP address of a DC and the domain it operates in, you will need to create SUBNETS in the Active Directory Sites and Services so that the DC knows it is supposed to respond to machines making requests from the subnets. This is how you manage SITES in Active Directory, by SUBNET.
No -- you don't have to use DHCP, but you'll regret it I am sure. It is a really nice way to keep track, even if you are using reservations.