Most of the European carriers can do this very easily, as the actual seats are the same in Y and C class. On most of the carriers, the only thing that changes is the seating on the left side of the a/c (ABC side) - in Y, there are 3 seats, and in C, there are 2. To do this, there's normally a lever on the side of the seat that, when moved, converts the centre seat into a smaller space, making the A and C seats wider. This takes just a few seconds to do.
You see this a lot on the high-revenue routes like LHR-DUB, almost all of which are operated by A321s - in the morning and evening flights, there may be up to 21 rows of C class, with just a few left for Y, and during the middle of the day the C seating may be reduced down to 5 or 6 rows since there's less demand. This changeover can be done during the normal aircraft turnaround time.