Ok so essentially this has a bunch of moving parts to it
Please let me know if you have an exchange server in your environment as that could add a step or two.
Prep your Active Directory forest for 2008r2 to do this you need to do the following on your domain controller
1) Put the Windows Server 2008 disc in the server
2) Open a command prompt
3) Explorer the Server Disc and open sources folder and find adprep.exe drag it into the cmd prompt If you can't drag 'n drop, you can simply type the path
4) In the Command Prompt window, type the following command: adprep /forestprep
5)In order to prevent accidental running of the command, you must press the "C" key on your keyboard, then press Enter. Command will begin to load a bunch of LDIF files containing all the necessary changes to the existing AD and Schema. Process will take a few moments.
6)When done, you'll be prompted. Make sure you let the existing Domain Controllers replicate all the changes throughout the entire forest BEFORE proceeding to the next step. However since you only have one this shouldn't take a long time
7)Since all your roles for Active Directory are on the same structure, you have to run it again to do the infrastructure prep its very similar to the forest prep you open the command prompt drag the file and this time in the command prompt type in adprep /domainprep this will take far lest time
Once you do this your domain will be ready for a windows 2008 R2 Domain Controller
Then you go through the process of making the Windows 2008 R2 server a domain controller. By running dcpromo on the box
You will now have 2 domain controllers you will want to run the Active Directory Sites and services and replicate the active directory between the two servers.
This will get the data over for group policies and any other data for Active Directory
Then you need to transfer all the active directory roles over to the new server
Setup DNS on the new server and have it sync with the other DNS server with Active Directory this should get all DNS records over to the new server
Go to the old server and make note of any DHCP Records that are static that you may have to recreate.
Next thing you will want to do is then run dc promo on your old domain controller so its n longer a domain Controller
Then unjoin it from the domain
Change the IP Address of the new server to what the old server was
Then on the 2008 R2 box you need to rename it to match the name of the old box
The steps to rename can be found here http://www.petri.co.il/rename-windows-server-2008-domain-controllers.htm
Once the server is renamed you will want to setup DHCP on the new server. And then reboot all devices to get the information from the new server
I would highly advise doing this on a weekend so there isnt a big impact on the office