Can I add the HeatRelease cooling later, or does it have to be bought with the mattress?

On the Hybrid Pro, HeatRelease is sold as an optional extra at the time of ordering: it is a graphite-infused layer built into the mattress, not a topper you can add later. If you sleep hot, decide before you buy. Our view is that the Pro already sleeps reasonably cool thanks to the wool layer and the airflow through the springs, so most people do not need it; the Hybrid Ultra, which has HeatRelease as standard, is the Simba for people who overheat badly.