MillionMileMarker

I'm having the issue that my flight from MEX to LHR was cancelled yesterday and moved to precisely 24 hours earlier.

I'm given the options to accept the flight change, find another flight or get the voucher - I of course want to keep the 50% and so pick a new flight.

However every date I've tried in december/January tell me that no flights are available - dummy cash booking says there is something available.

Edit: I have checked the redemptionfinder and it does not even let me rebook on dates where there is reward availability (in August for example).

Any advice, guess I have to call, but wanted to check that I should be able to rebook for free in these circumstances?