You'd almost certainly have to perform a mid-contract upgrade over the phone, and it always seems like it depends on who you talk to and how their day is going. If you were to upgrade mid-contract, expect .to pay the full price from the published price list and not the "discounted" new customer rates.
Even in a flat, if it has thick solid concrete walls, you might be better off with a powerline/WiFi extender than SuperWiFi.
