Yes, the remaining legs on your ticket would definitely be cancelled. As an alternative to rebooking your flight according to your actual itinerary: have you considered flying to Y on the inbound as currently booked and then finding another way of travelling back to X? By train, one-way fligbt, car, whatever? This may well be cheaper. If this would lead to a ti.e problem, you may wish to start your trip a day earlier if possible. This may still be cheaper than going for the expensive ticket.
​​​​​