I replaced a moderately high end mattress after ~15 years. It was perfectly fine, just wanted a bigger bed. Just bought from a local bedding manufacturer. It wasn't cheap but wasn't top of the line either. Just firm with a small pillow top. I put an 1.5" memory foam pad on top of it since the date of purchase. My new bed is ~1.5 years old, made the mistake of getting a 3" memory foam since 1.5" was great 3" will be 2x as great right? OMG so hot. I've figured it out mostly but I'll be getting another 1.5" topper if I ever get off my lazy rear :-)

So good box springs, I put a fitted non deep pocketed sheet between the bed & foam, then a deep pocketed over the foam and whole bed. Chuck the foam every few years and the bed seems like new. Plus that Kool-Aid spill just went into the memory foam! lol O:-)