Hi SouthFL,
Before you do the exchange I would take the precaution of making sure that any sagging or softening is coming from the mattress itself and not from either your support system under the mattress or from the wool topper over it. You can test this either by putting the mattress on the floor (to test the support system) or by sleeping on it without the wool topper.
Once you’ve confirmed that it’s the mattress itself … then post #2 here and the posts it links to has more information about the two approaches you can take when you are exchanging a mattress for another one and your options are limited to what is in the store.
If you look at the specs of the Prana Vinyasa here you can see that it has 2" of lower quality polyfoam in the quilting layer which is not great, especially for a mattress in this price range, but it’s also “not too bad”.
You can see the layering of the Beautyrest Black Brooklyn here and as you can see there is about 6" of lower quality materials. I don’t know the densities of the foams but you can see a similar Beautyrest Black mattress here (that’s about 1/2" thicker) and as you can see there is a lot of lower quality/density materials in the comfort layers and the whole line would be similar and I would tend to avoid it.
I would also want to know the details of the materials in the Mary Beth Pillowtop but in general I would tend to avoid any pillowtop mattress made by a major brand (unless you can confirm the quality/density of every foam layer in the mattress) because there would be too much lower quality materials in the comfort layers. You can see in the very general description here that there are at least 5" of questionable materials in the Heidi Pillowtop.
Phoenix