You don’t need to replace the old mattress if you get this. “fluffy” is the real word for it. It’s thick.  My daughter is just loving it how soft is her mattress now. I got it just 2 weeks back but by the look of it I can say it will go a long way. It stays in place even without fitted sides. doesn’t slip like cheap mattress toppers. Bedsheets kind of stick to it that’s another plus. All in all I’m very happy with this purchase.