BRUpax
Equipment change intra-fleet and externally is not uncommon, many reasons neccessitate this from time to time as I'm sure you know. However, this does not affect your rights. Segregating J or C class pax in Y class seating does not meet the criteria for the fare you purchased, you were what's known as 'involuntarily downgraded'and are entitled to the fare difference between C and Y, This is standard IATA practise to which CY have not excempted themselves. Most carriers also offer downgrade compensation as away of apologising for the inconvienience, C class is high yield traffic and carriers usually like to protect that. Suggest you contact the nearest local CY sales office and request your entitlement.