Bernard,

I had a similar problem. My morning flight to Dubrovnik was cancelled and I was put on the lunchtime flight (the only other flight of the day).

When the morning flight was reinstated, I asked to be put back on it, but was told I had 'accepted the change'. Still not sure what I did to confirm my acceptance!
Eventually they saw sense, and 'because I had hadn't any other option at the time, and as a good will gesture' they changed my booking.

I'm not sure what game they're playing, but I'd be putting in a formal request for EU261, and as nufnuf says following up with a MCOL if necessary.