novakom, I wouldn’t use a topper on the old mattress. It won’t solve your problem with a worn out, sagging mattress. It can allow you to soften up a mattress that is too firm, and give you a more durable foam in the comfort layer. I gather your bed is a queen, as the king Seven Comforts is in stock. In the meantime you might look at post #9 here and a link from Phoenix in a following post to see some ways to get a little more life out of the old mattress until you figure out what to do.
https://forum.mattressunderground.com/t/denver-mattress-snowmass-softened-in-4-days
Of course, you have to be a serious tinkerer to do what I did with those boards 
Sometimes the magic works and sometimes it doesn’t…Lew