Hi jaybackintheday,
This would be accomplished either by replacing a layer with another one that was the same thickness with something that was softer or by adding additional thickness/softness in the form of a topper.
This is certainly possible and I tend not to argue with someone’s personal experience
I don’t know of anywhere that you can currently buy a 7 or 8 lb memory foam topper.
I would also keep in mind that most 4 lb memory foam will tend to feel softer than most 5 lb memory foam although this isn’t always the case. Post #2 here and the topper guidelines it links to may help you use your actual sleeping experience as a guideline to help you decide on the type, thickness, and softness of a topper that you believe may work best for you.
Phoenix