Back problems are not all alike, but...
You just might want to rethink this one. An upright position means any road shocks go right up a spine that is in a poor position to absorb them. A more horizontal spine generally has a much easier time. To pull off the more aggressive position does require that you maintain good core strength and that you ease into the lower position gradually.

I know that I had pretty serious back problems in the past and tried an upright posture for a while. In my case it was a big mistake and I almost gave up cycling until figured out that I needed to slowly worked my way back to a more aggressive posture. I went from thinking I could no longer ride to doing a coast to coast and other longish tours and NOT using an upright posture was one of the key things that allowed it.