hi RoxCo,
Unfortunately I think you hit the nail on the head. If you buy a mattress that has thick layers of lower quality polyfoam in the upper layers such as in a pillowtop … and it is just inside the “range” that is acceptable for you in terms of spinal alignment … then even the initial softening over the first few weeks can put you “over the line” and the mattress may no longer be suitable for you (even though it may still work for someone else with a different body type or sleeping positions). It is unlikely that there is anything here that would qualify for warranty coverage because loss of comfort and support from foam softening is not a warranty issue (even though it’s the most common reason people need to replace their mattress).
You could either sell it (and used mattresses don’t command a very big price) or you could do “mattress surgery” which means carefully cutting open the cover, removing all the “junk” foam and replacing it with higher quality foam, and then either re-attaching the cover or buying a new zip cover to enclose your “new” mattress. The springs will still be fine … it’s just the top foam that has softened too much for your needs and preferences.
If you sell it … I wouldn’t repeat the same mistake and walk into a chain store and only buy another mattress that is low quality and overpriced for what is in it. These guidelines will help you avoid most of the worst choices when you are mattress shopping.
Unfortunately $5 - 600 and “quality” (depending to some degree on the size you are considering) don’t really go together and I would be going for “the best available in the price range”.
Some of the better options in the NYC area are in post #2 here and a more detailed description of many of these is in post #4 here.
The size would make a big difference in what was available inside your budget and if you let me know the size you are looking for then I could probably help you narrow down the list a little bit more.
Phoenix