This reminds me of when I first brought my dog home from the shelter. She must have lived in a one-story house before coming to the pound, because she couldn't walk up or down stairs! If we really talked her through it, she could run up the stairs, but then couldn't figure out how to walk down. I would have to pick her up and carry her down and she would be moving her back legs as if she was actually walking.
Obviously we trained her to walk on stairs (thank god!) but it was definitely a fun transition period :)