Replacing a box spring when buying a new mattress is only necessary if the old box spring is in bad shape. If only a new mattress is needed, it is not necessary to purchase a new box spring if the old box spring is in suitable condition.
Purchasing a new mattress usually fixes problems the customer is having. If the box spring can still support the new mattress without sagging, it is ideal to keep the old box spring. Box springs are only worn out if they are cracked, ripped, sagging or if they have any problem that prevents them from giving support.