It is that way because they are two different setups and transferring a configuration from the Capsule to an Extreme or vice-versa they are just two different kind of setups because if the internal hard drive! If you transfer now expect setting up problems in the future. It doesn't take a whole lot of steps to configure a wireless router.
Like I said before set it up manually to prevent the different from the setup of your older Capsule! So just writing down the setup to use to setup the new Extreme will prevent future problems!