It is not necessarily required to replace your box springs when purchasing a new mattress, but it is recommended to do so if your current box springs are worn or damaged. (100% true but it's often difficult for a consumer to tell when it's time. Our delivery team are experts and will let you know when they deliver your new mattress whether your foundation needs replacing).
A new mattress can put extra stress ( while it's possible a new mattress is heavier than your old model it's very unlikely if your existing one is more than 25 years old because back then all mattresses were made two sided) on old box springs and may not provide the proper support for your new mattress.
Additionally, older box springs may be worn out ( although all manufacturers used to put way more technology into their foundations (25 years ago) but it's more likely if you purchased your existing box spring in the last ten years) which can affect the comfort and support of your new mattress. If you are unsure whether your box springs need to be replaced, you should consult with the manufacturer or a professional in the mattress industry.