You don't always have to buy a new box spring for your new mattress unless the box spring is torn or has broken slats or any other parts broken in it.
Personally whenever I replace my mattress which is every 12 years I go ahead and replace the whole box spring and mattress together but you don't have too.
I also replace the entire bed by then because after 12 years I just want a new bedroom set instead of keeping the old bedroom set.
You only live once so indulge yourself in something new after awhile.