Hi citronlady,
Unfortunately you are in a difficult position as far as a mattress exchange goes. Some of the suggestions in post #66 here and a couple of other threads it links to about members here in a similar situation may be helpful.
Almost any “all foam” mattress (using latex, polyfoam, or memory foam) can be used on an adjustable bed and there are also some innersprings as well that are designed for use on an adjustable bed as well. If you have any doubt then I would check with the retailer or manufacturer who will tell you if they are OK and whether the warranty coverage would be maintained.
I would pay particular attention to the layering of the mattress itself because whether it works with an adjustable base or not has little to do with whether the mattress is suitable for you in terms of what I call PPP (Pressure relief, Posture and alignment, and Personal preferences). The suitability of a mattress for the needs and preferences of any specific person has to do with the specific design of the mattress and the layers in it rather than who manufactures it.
In general terms … your best option may be to select a mattress which has the least possible amount of polyfoam in the comfort layers and then add a more durable topper to this to create your own comfort layer. This will give you higher quality comfort layers than are typically found in either Sealy or Stearns & Foster mattresses but will also extend the life of the base mattress as well (there is more about this in the posts I linked).
If you provide a link to the store where you made your purchase I can make a quick scan to see if any of their mattresses stand out as being more suitable even though I normally wouldn’t recommend any Sealy or Stearns & Foster mattress. It would also be helpful to know the size and the price of the exchange mattress you want to stay under when you make the exchange.
I should also mention that if the base you tested the mattress on in the store was a firm non flexing base … then it would have a similar effect on the feel and performance of the mattress to the adjustable bed in flat position (both are firm non flexing bases) and the mattress may just need to go through its initial softening period to feel like the one in the store. How long have you slept on the Optimum? Another possibility is that a mattress protector, mattress pad, or even the sheets and bedding you have may also be affecting the feel of your mattress compared to what you tested in the store.
Hope this helps … and if you have more questions about your rather unfortunate situation … feel free to post them. With some better choices and depending on what they have available as a mattress replacement … you may at least be able to make the best of a difficult situation.
Phoenix