How to replace FBX Objects in the projects folder properly?

Hey, we are working in a team of three and syncing over Dropbox. We have a FBX File, for example Monster and 3 fbx animation files called monster@run and so on. Then i have a normal prefab called Monster, which i use in my scenes. There are various scripts on this prefab.

Now i want to replace the monster file and the animation files in my project, but i cant simply drag the new fbx files into the project and delete the old ones. So i swapped all the files IN the monster of the hierarchy with the ones from the new FBX and applied then to update the Prefab.

So i synced the everything and now we lost all the connections to the meshes and textures. So my monster is invisible^^

How do u solve this problem when u want to replace something??

I drag and drop in the Finder, option-dragging the original files elsewhere, if I need a copy. Unity doesn't know the difference between this, and an edit of the original file.