When replacing components in assemblies, if you select Re-attach mates, the software can access previously assigned face and edge names in lightweight components.
If you have previously assigned names to mated entities (faces or edges), then when you replace one component with another, you can reattach the mates automatically without resolving the original component.
Previously, the mates would not reattach automatically because the software could not access the entity names in lightweight mode.
You must save the assembly in SolidWorks 2012 to access the face and edge names in lightweight components.