By "first version" are you referring to the 802.11g (dome-shaped) Extreme or the 1st generation 802.11n (square-ish) model? Will you be setting them up with an iOS device or a Mac/PC? I am assuming that you will be using the latest version of the AirPort Utility as this is the only version that will support the 802.11ac AirPort models ... is this correct?
Ok, thanks for clearing that up for me.
I would recommend that you perform a "factory default" reset on the 1st gen Extreme. Allow it to restart.
Your iMac should now "see" it in the AirPort drop-down menu as a new base station. Select it from the menu and the AirPort Utility will start up and take you through setting it up.
Just follow the setup dialog to configure this base station to extend your new 802.11ac AirPort.