Thanks, Justin. That was really helpful. It took some work to figure out how to unlock the domain in wordpress.com, but I think everything went smoothly.
Here's a question on whether, and how, the two data sets will be merged when we transfer the registration for this domain. My wife started a blog at novalibrarymom.com and her wordpress.com dashboard still shows posts 1-76. Then we transferred the hosting in late March 2012, and the first post that shows in her wordpress.org dashboard is what she calls #77.
So when we transfer the registration, will the two data sets be merged, and she'll see posts 1 through n, all in the same dashboard? I think the answer is no, which is why we need to export her old posts and import them into the wordpress.org architecture running on DH. But then if we import the exported files, won't that write over what's already there? Is there any way to append to the data set instead of overwriting it?
Again, thanks for your time and help. Like I said, I know just enough to get into trouble but not enough to get out.