MCE is officially just an economy seat with a paid seat selection, so if you take a voluntary change they are not "downgrading you" if they cannot or do not provide it on the replacement flight. For EU261 purposes I don't think it counts as a downgrade either but I'm no expert in that regard.

And since the replacement flight would be fitting you onto a plane at the last minute, odds are all the MCE will be filled up from purchases plus elite members getting free selections at check-in, so don't count on it.