To decommission an Active Directory Domain Controller (Windows Server 2003/2008) is a fairly straightforward task so long as you make sure nothing is relying on that server specifically. I will cover off the demotions steps (fairly easy) but I will go through a checklist (and how-to) of gotchas that you might get caught out when doing this.
UPDATE: 27/09/2019 – Quick update to say that this is definitely the most popular comment on my blog. Thank you all for the all the comments, and please do keep them coming. The more comments i get, the more likely i will start to write articles again.