This is a topic that is always rather subjective, AD In-Place Migrations… One of my colleagues [Rick Claus] has been blogging about his experience over the years with just this sort of thing:

When looking at the migrations, there are often many questions you will be asking yourself and tasks you will be performing, from identifying what you have now and where you would like to be, to looking at logical and physical design components for AD. Whenever I have done migrations, one of the biggest challenges is often just getting everyone’s buy in!

Rick has done a great job in a series of articles give the benefit of his years of experience doing this for other companies large and small. I would suggest taking a look if you are either planning a migration or have already done so. He has a host of tips for you.

Oh, and please give him your feedback J