Replace 3D Assets with different File Type?

I'm currently moving an older project from Mac to Windows, and ~90% of the mesh assets are in Cheetah3D .jas format. Exporting them to .fbx is trivial, but there appears to be no way of making Unity Editor swap out the old Asset for the new one.

Most of the assets are not dedicated prefabs, but just the model file dragged into scenes, with pieces removed, duplicated, reparented, varying materials applied, etc... Tens of thousands of times.

I just need to tell Unity to use the .fbx instead of the 'missing' (incompatible) .jas file (and keep everything else in-tact).

AssetDatabase scripting will allow renaming the asset, but not changing the file extension.

Is there any way of accomplishing this?