Reconnecting Source Files (and 'Import Into Level' issue)

Within my scene, I have used ‘Import Into Level’ (which I believe is 4.12), combining meshes into a single blueprint.

I’ve had this happen many times where the source file will either mistakenly reference another FBX or simply not be there at all:

Is there a way to relink the source file when this happens? If I try and reimport through any conventional means, it creates duplicates and loses some saved information I needed.