The FTTC plate maintains the existing phone socket and adds an additional socket for the modem.MrSaffron's web site has pictures of the FTTC filter
that show how it fits together. If you click that link, Heyokah, you'll see that, as Ribble explains, there should be no need to change your setup.
I have Zen FTTC and am very happy with it. It's a question of weighing up the pros and cons - Zen has a 12 month minimum contract period (the same as Openreach imposes on the underlying wholesale FTTC connection), UK based support and static IP (you'd keep the same IP address(es) you have now if you migrate your existing ADSL connection to FTTC). BT Infinity has a longer minimum contract period (18 months, I believe), support that is largely offshore and dynamic IP. However, BT Infinity includes BT Sport and the Home Hub 5.
I don't believe there's a way of setting up the Home Hub 5 as a transparent bridge, like an Openreach FTTC modem. The Openreach modem should connect directly to your Airport Extreme.