When upgrading a project from Unity 4 to Unity 5, you may need to re-bake stored data because some of the prebaked data formats have changed.
Baked Occlusion Culling data format was changed. Rebaking is required for the occlusion cullingA Unity feature that disables rendering of objects when they are not currently seen by the camera because they are obscured (occluded) by other objects. More info
See in Glossary data.
Baked Lighting data format was changed. Rebaking is required for the lighting data.