Unity uses .meta files for each asset you add to a project. These contain a GUID (unique identifier) and additional information about _how_ an asset should be imported, e.g. max texture size, compression settings, … everything you change in the importer. All the information required to reconstruct the Library representation of an asset is stored in the meta file.

• .meta files are important: always check them into source control, or you'll lose the ability to restore assets if you delete the library.
• .meta files are powerful: they allow you to _replace_ assets if you know what you're doing.

Note: please make a commit / have a backup before attempting any .meta file magic.

or: how to replace a file without having to rewire everything

Say you have added a PNG file to your project and used it in a dozen places. Later, you realize that you actually need more control and want to “upgrade” to a PSD file. Usually you would have to

• add the PSD file
• find all places the PNG file is used ← this is slow and error-prone
• use the PSD file instead
• delete the PNG file

But, with a bit of .meta file magic, you can replace the file directly and keep all references . Note: do not focus Unity during this procedure, otherwise .meta files will be created where you don't want them.

• right click your PNG in Unity and show in Explorer/Finder
• there's a “MyImage.png” and “MyImage.png.meta”.
• put your PSD file here; usually “MyImage.psd” but can also be “MyFancyImage.psd”.
• rename the meta file to now point to your new PSD file: “MyFancyImage.psd.meta”
• delete “MyImage.png”
• focus Unity

That's it - the image is replaced everywhere it's been used and now points to the PSD file. This works because

• the GUID is stored in the .meta file and used for tracking the reference
• the import result of both files is the same (a texture), if you replace e.g. a texture with a mesh stuff will break.

This also works for e.g. swapping out an OBJ file with a matching FBX, etc.

This is a bit more involved and will only work in specific cases. _Prefabs_ in Unity have both a guid (which identifies the prefab) and a ton of fileIds. Every object (GameObjects, Transforms, Colliders, …) have their own fileId. Assets in the scene are always referenced with (guid+fileId) so that there's a clear link.

So, when you want to replace a prefab, you have to

• replace the guid in the .prefab.meta file
• replace at least the root Transform fileId in the .prefab file (this is used to place the prefab in scenes)
• replace all other fileIds you want to keep intact (e.g. objects reference stuff inside the prefab, overrides you have applied to specific things, … be careful)

or: a real-world example why you would even want to do the above:

When you're careful and have defined a “Prefab Interface” as outlined above (matching GUIDs and root transform fileId between objects), you can use this for something very powerful: You can switch out all your high-resolution project assets for low-resolution project assets.

Note that these can have different meshes, different textures, … all that matters is that they're “dumb”, that is, are only placed in a scene and not have further overrides / modifications / links.

A good way to do the above is to have two Packages, com.me.high-quality and com.me.low-quality that contain the same prefab interfaces and otherwise all the meshes/textures required.