Annie,
I have no actual experience with this yet...but here is what I'm planning to do.

(So this is still total trial-and-error on my part, not to mention that every child is different.

)
I'm new to HOD this year. I started with LHTH and loved it so much, that I found the HOD website and their programs for older kids. I'm going to start Bigger with my ds...soon. Since he has never done HOD, I'm going to make our own "book pack" from the Book Shop. If you look at the books in the level 2 book pack, it looks like they range from 3.0-4.2 on reading levels. (Third grade, 1st month - fourth grade, 2nd month)
Carrie has a book list with lots of books to chose from in the different genres. (I don't have it yet, so I don't know if it has reading levels, but I would think it would.) So my suggestion would be to pick books from the genres that you think your daughter would like that are at about the same reading level that the emerging readers are, so you can ease into DITHOR.
With my ds, I'm going to pick some of the books that were maybe used as read-alouds for Beyond or others that we didn't read that I think he would enjoy. You could do this and purposefully keep the reading level at what you think she can have success with. I know it's also good to challenge them once in awhile so they can move into something more difficult, but if you're worried about the transition, start with something you
know she can read, so she enjoys it!
I hope that made some sense...I've been interrupted here.

Kathleen