Am not 100% sure but I feel that you can cancel one part of the flight as they usually say the two flights are completely separate. They will send you a second notification if you do not respond, so I would not worry about a few days. I have had a few flights times changed with Ryan Air to Malaga as they cancelled the 17.10 flight. What I did was ring them and I changed all my flights to suit myself. I just did that yesterday, I had booked for early September but I rebooked for the end of Sept as the end of the month is better for me now. I just gave the lady the new dates and she changed them no prob and no extra charge! As far as I know the phone call is charged at the National rate not one of the expensive numbers. (At least I hope so!)