Married segments = changing the HEL LHR means checking availability from HKG to LHR via HEL (or perhaps even SYD) so it depends on availability for these flights. Even if HEL LHR is available in I and you have HKG HEL already booked in I, if at the moment you request the change there is no I class available for (SYD) HKG HEL LHR, you'll need to pay for the fare difference.