- to replace the OBJ: Edit -> Project Configuration -> 'Select' button. If you have painted manually, this only works correctly if the new object is in the exact same location in 3D space as the old object (bounding box cannot change).
- to copy exisiting layers to a new project: put all layers into a new group, name the group, then create a smart material from it (right-click the group -> Create smart material). Open the new project, then drag-and-drop the new smart material into the layer stack.