Hi Gonzo72,
Welcome to the Mattress Forum! 
I’m sorry about you not liking your Casper mattress. I’ll do my best to provide some suggestions based upon the information you provided, but unfortunately I’m unable to diagnose specific back/pain issues via an online forum.
First off, there is some information about the many different symptoms that people may experience on a mattress and some of the most common causes behind them in post #2 here that may be helpful.
You didn’t mention how long you tested out your Casper mattress, as everyone has a degree of “learned alignment” that they generally need to overcome when trying out a new product. Hopefully you tried out the mattress long enough to rule that out.
I’m not sure exactly where in your upper back you’re referencing the pain, as the ribs articulate the 12 thoracic vertebrae, but if you’re experiencing pain in the upper part of that region along the lateral borders of your vertebrae where your ribs articulate, that could be an issue with your pillow being too thick for your new mattress setup, placing stress on your upper back and erector spinae group. It could also be an issue that you are sinking more than you desire or are used to, and this could also cause you some issues. It could also come down to your own personal sensitivities or level of flexibility/strength. Or it could be a combination of all of these things, among others.
In cases like this, your initial hunch is usually quite worthy of consideration, so as you mentioned you may want to investigate something that has a bit more surface resilience, but still with good contouring properties. Latex certainly can be a good alternative to memory foam in this area, as latex is very point elastic (contours well without pulling from the surrounding area), but it also can be a very supportive material, whether in a combination with al all-latex mattress, one using innersprings, or even one using a good quality polyfoam core.
If you do consider Sleep On Latex, they are a member of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. They are extremely knowledgeable about latex and different configurations, and I would not hesitate to have a detailed phone conversation with them to explain your past experience and receive suggestions that they may proffer.
Phoenix