This seems to be a problem with many airlines. I just got a notice by email from WizzAir that my flight into London for a UK timeshare exchange was moved from Sarturday to Sunday. Wizz only flies that route 6 days a week once a day. For a Saturday to Saturday timeshare, Sunday just will not do, so my choice is 1) rebooking on Wizz for Friday and renting a hotel room for the extra night, 2) changing my departure city to another one that has daily flights on Wizz to London which would involve additional transportation to get to the departure city, or 3) getting a refund and using BMI miles one directiion and a sale fare with another airline from Wizz the other way (OW here is often economical, unlike in the states, and BMI allows OW award bookings for half the miles). Given the looming BA takeover of BMI and the resulting devaluation of miles there, my inclination is #3.
But it can work out to your advantage. RyanAir rescheduled one of my flights on a previous timeshare exchange to a flight that would not have worked well. However, due to demands at work, I had cancelled the exchange and was expecting to just eat the air tickets. With this change in schedule, I was able to get them to refund a non-refundable ticket.