Simply Put: Probably. But there are circumstances in which your old one may be okay.
For a number of reasons, many manufacturers do not honor their warranty if a mattress is purchased separately and used on an older foundation. However, if warranty is not a concern and you really want to save the money, here’s the scoop.
There are different types of box springs, or foundations. Some are designed with metal modules inside that act as shock absorbers. These have a bit of flex and do fatigue over time. Putting your new mattress an old “box spring” will likely cause premature wear of your new mattress. Push down on the corner of your box. If it has any give, you should replace it.
“Foundations” are more rigid boxes, having a stiff surface with little to no give. If your foundation is of this type, be sure the surface is completely flat with no dips. If it feels sturdy, doesn’t squeak, and still has a straight flat surface, it may be okay to use for your new mattress.
You might also consider the fact that your bedding is a haven for dust mites and allergens. If you are prone to allergies, or if the thought of microscopic things crawling around in your bedding gives you the creeps, you may sleep more comfortably knowing the old stuff is gone.
The bulk of the investment is in the mattress, so generally there isn’t a large price difference to go ahead and get the complete set.