How to swap out a child object with another, preserving the parent inverse matrix?

I have a blend file which links an object from another file (e.g. a pistol). This object is part of a larger model (a person, who is holding this pistol). If I update the linked object , in the linked file, eg add child objects to it, the copy in my main blend file does not update. So, I need to replace the existing linked child object with a new copy of this linked child object. But, there are animations on everything, so I need to preserve the Parent Inverse Matrix. How to do this?