In an assy, you can multiple select different parts and change to a new config on all of them at the same time. The key is that the different parts all need to have the same config name that you want to change to. When you open the properties box, only the config names that are common to all of the part files selected will show up. This means that if you are setting up a project, keep in mind that you might want to be consistent with your config naming, such as each file has a config called 8A, 8B, etc.
WT