I have an AirPort Extreme home network that is extended with an AirPort Express. Because some legacy devices within the household do not support WPA/WPA2 Personal, I would like to keep my AirPort Extreme network secured with WPA2 while allowing devices that can connect to the AirPort Express to connect via WEP. Is this possible? (knowing in advance would certainly help, as I do not want to mess up the current configuration for the AirPort Express.
First you can save you current configuration in Airport Utility.
Then you have two options: 1. If your Extreme support guest network then just set it up as WEP and extend it with Express. 2. Or connect Express with ethernet to Extreme(i.e. just use it as gateway to internet) and setup WEP network on Express.
This is admittedly not a full answer. But I am going to bet that your final method is going to involve having your airport Extreme serve up two separate networks (you can configure it to serve up the 5GHZ band as a fully separate wireless network.) But, this has the obvious disadvantage that that network will not be repeated on all the airport Expresses.