We have a "too firm" Sealy mattress, and I decided to try something. I researched memory foam mattress covers and finally decided on a somewhat dense 2 inch thick memory foam. I had to double check everything because most are 58" for a 60 inch wide Queen. I did not want that. I finally found one that was a full 60 inches wide. My wife and I have enjoyed that for 8 years. We both needed firm support but wanted something soft and cushiony. The dense 2" memory foam on top of a firm mattress has made our sleep excellent for years.
That is probably not for everybody.
BTW, I know that Casper Mattress does/did this, and some other online orders may allow for this: If you try it for a week or so (read their instructions) and do not not like it, call them, they may tell you to contact a Good Will Store, or Salvation Army or other store or organization that helps those in need. Donate it to them, get a receipt and the mattress company will refund you the cost of the mattress.