Hi Suec,
Unfortunately the issues you are having are much too common because the major brands (including the Beautyrest Black line) use some low quality materials in their mattresses that are much more subject to premature foam softening and impressions although you are somewhat fortunate if you have had your mattress for 8 years because many others who have the same issues after only a few years. You are also fortunate that you qualified for a warranty replacement because many warranty claims are denied because the visible impressions aren’t deep enough (the mattress has lost it’s comfort and support because of foam softening or “virtual impressions” rather than because of visible impressions) or because there is a stain on the mattress.
Having said that … like many of the members that have come here with a similar situation you are in a somewhat difficult position where you need to exchange a mattress and may be “locked in” to a store or a specific manufacturer that sells mattresses that I would normally suggest avoiding in the first place and where there may not be any particularly good quality/value options available to you.
If this is the case there are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation.
Since you already have a good quality latex topper it may be worth considering a mattress that is firmer than you would normally choose and then using your topper to provide the additional softness and pressure relief that you need.
I would also keep in mind that as long as the materials in a mattress you are considering are durable enough for your body types and meet the quality/durability guidelines here … the choice between different types and combinations of materials and components or different types of mattresses (see this article) are more of a preference and a budget choice than a “better/worse” choice. Testing different types of mattresses that use different types of materials and components will be the most reliable way to know which type of mattresses you tend to prefer.
There is also more information in post #3 here and the posts it links to that would be helpful for those that are in higher weight ranges (and/or that sleep with a partner that in a lower weight range and has different needs and preferences)
While the process of how to choose a mattress would involve the same steps that are listed in the mattress shopping tutorial … most people in higher weight ranges will generally need or prefer firmer mattresses (firmer materials will feel softer because you will sink into them more) and materials and components that are higher quality and more durable than those that are in lower weight ranges (the materials and components in a mattress will soften and break down faster for those in higher weight ranges than they will for someone that is in a lower weight range that doesn’t compress the mattress as much). I would be particularly cautious about mattresses that use more than “about an inch or so” of memory foam that is less than about 5 lb density or polyfoam that is less than about 2 lb density … particularly in the upper layers of the mattress.
I’m not sure if this will be helpful since you are probably limited to a specific retailer or manufacturer but the better options or possibilities I’m aware of in and around the Toledo, OH area (subject to making sure that any mattress you are considering meets the quality/value guidelines here) are listed in post #2 here.
Phoenix