Hi anotherscott,
I advise against buying a new mattress with the intention of adding a separate topper afterwards unless there is no other reasonable alternative or unless you have a chance to do some careful and objective testing for PPP (Posture and alignment, Pressure relief, and Personal preferences) on the specific combination in person using the testing guidelines in the mattress shopping tutorial. Deciding on a topper that would be a good match for both you and the mattress (the specifics of the mattress can make a significant difference in which topper would work best for you) can be almost as challenging as buying a new mattress in the first place. And if a mattress is already in a “soft” configuration, as was this case, it may be that the componentry within the mattress is incompatible with the comfort desired, and adding a topper would not provide an appropriate comfort, or could result in too deep of a “comfort cradle” and negatively impact alignment, especially when sleeping prone, as in this situation.
If someone has an existing mattress that is still in good condition and that they consider a bit too hard, a topper can be viable solution to increasing the plushness of the product without negatively impacting alignment, and the guidelines in post #2 here and the items linked therein provide quite a bit of color on that topic.
Phoenix