Hi RemiRoo,
Like some of the other forum members here you are in a difficult situation where you have to exchange one mattress that uses questionable quality materials for another that may also have the same issues. In most cases the problems you are facing is because of the use of thicker layers of lower quality foam in the upper layers of the mattress (which is the weak link of most mattresses) and this is a common issue with most larger manufacturers (who tend to focus on maintaining or increasing profit margins and mattress turnover more than the durability of the materials in their mattresses) and most of the mattresses sold at mainstream chain stores as well (which is one of the reasons I normally suggest avoiding major brands and chain stores).
Post #2 here includes some links with good information and conversations with other forum members in a similar situation that you may find very helpful in choosing a mattress for your exchange (including another forum member who is currently working on a “major brand exchange” as well).
Post #1 here would also be helpful and has the basic information steps and guidelines that can help you make the best choices even though part of it wouldn’t apply to you because you won’t have to research different manufacturers or retailers because you are limited to a single store.
The key is to make sure the mattress uses high quality and durable materials in the comfort layers or alternatively to minimize the use of lower quality or unknown materials and then add your own comfort layer in the form of a high quality topper.
Hope this helps … and once you’ve read some of the linked threads, if you have any questions along the way feel free to post them.
Phoenix