Hi,
This is done on purpose so that the customer has less things to fill in the form. It is a new address form, but with the fields prefilled so that the customer just has to change the information that is different for the new shipping address.
Now if you want to change that, it is still possible with some code modification but we won't change that as that's the legacy checkout you're using. The new checkout system (which you can use by turning off the "checkout legacy" setting of the HikaShop configuration has a new address system completely redesigned to make it less confusing and avoid such issues.