When a walk-in takes over a prior human’s physical body, do they inherit the interdimensional DNA of that prior soul or do they bring their own interdimensional DNA with them and inherit solely the portion of DNA already present in the physical body?

This is an important distinction in the function of the DNA in its two major types. The interdimensional portion of DNA is an extension of the soul and comes with the soul when joining with the physical body. It is akin to another energy field, so a walk-in will bring their interdimensional DNA to make the connection to the physical body. The departing soul extension will withdraw the interdimensional DNA and make room for the new inhabitant.