Just called and rebooked my open ticket via the call center, and it was very easy! I was a bit worried because of the Europe 1 and Europe 2 in the award zones, but it seems all of Europe is considered one region for the purpose of rebooking. My original flights were GOT-IST-SFO, SFO-IST-OTP and I rebooked into SKP-IST-SFO, SFO-IST-LHR with no issues.