Are you asking whether your sites, files, databases and everything get moved over automatically or do you have to set them up all over again?
If so, then you've got nothing to worry about. The Dreamhost support staff responsible for switching you over will be responsible for moving everything over and getting everything running again. The only downside is that your sites and such will be down for the day or two this takes (this goes in parallel with the DNS propagation time).
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