Hi Hextejas,
The density of memory foam is the single biggest factor that affects durability and while “as a group” higher density memory foams tend to feel firmer (and can also have a slower response and be more temperature sensitive and contouring) … density isn’t a reliable indication of how soft or firm a memory foam topper will feel because softness and firmness and many other properties of memory foam can be chemically formulated into the memory foam regardless of it’s density and some lower density toppers are firmer than some higher density toppers. There is more about the many different properties of memory foam including density that can affect how soft or firm it feels in post #9 here and in post #8 here
When the time comes to replace your topper then if you were happy with the topper you have then it may be worthwhile just buying it again but if you aren’t happy with how long it lasted and you decide that it would be “worth it” to purchase a higher density topper that would last longer then there are so many options available that the most reliable way to choose a memory foam topper that you haven’t tested in person is to have a more detailed conversation with a knowledgeable retailer or manufacturer who can provide you with more information about how the memory foam toppers they have available compare to some of the other types of memory foam on the market.
Phoenix