Hi celldweller454,
Unfortunately your issues with your mattress are all too common and are almost always the result of a mattress that uses lower quality and less durable materials in the upper layers (especially with pillowtops) which will soften or break down much too quickly. While you may be one of the fortunate few that quality for a warranty replacement (and it’s certainly worth checking) … in most cases the impressions in your mattress will be less than the warranty exclusion (which is commonly 1.5") and the issue is more about foam softening (or “virtual” impressions") which results in the loss of comfort and/or support and the need to replace the mattress which aren’t considered to be a defect and aren’t covered by a warranty (see post #174 here). This is one of the reasons that I strongly suggest avoiding most major brand mattresses or any mattress where you can’t confirm the quality and durability of the materials inside it (see this article).
While it’s very difficult to fix these types of foam softening or sagging issues without removing and replacing the foam layers that have become too soft or are sagging … there are some suggestions in post #4 here that may be worth trying and may be helpful.
Phoenix