Mattress for Male back/side sleeper with bad back

I am 83kg, 5,11 and male. I like to sleep on my back initially and then move to my side after a few hours. I wake up with a bad upper back which can extend to the sides of my rib cage if I then sleep on my side for a while. My mattress is a pocket sprung with Visco e plastic comfort layer but it's 12 years old now. What do you recommend?