I was under the impression that after Ruby's spirit was sent away by Lilith and then she herself vacated the the body they had both been in that the human shell was dead. I'd imagine being possessed and vacated by a demonic entity would be extremely traumatic to go through once, let alone twice. Not to mention she had been shot at least once by Bobby a ways back.
The other actress might have been overpriced for their budget, but at least with the concept of "Ruby", we didn't lose the character. As I've only seen Lazarus so far, I'll reserve judegement on whether I like the "new" Ruby or not.