I have an existing home network using older Airport Expresses (b/g). They are running firmware 6.3. I just bought a new Airport Express to add to the network, which runs 7.4.1. Can I not update the older Airport Expresses to the new firmware? It seems to add some new features that I'd like to use across my network. For example, the 7.4.1 adds the option to "Extend this network", but I can't find it on the old firmware Airports. Any help?