This is debatable.

According to art. 9, the airline should provide food and a hotel room. If the airline doesn't provide this, then the passengers seek compensation from the airline for the airline's failure to fulfill its legal obligations (usually an amount equal to what the passenger had to pay to get food and a hotel room at his own expense).

According to art. 8, the passenger should be rerouted "under comparable transport conditions" and "at the earliest opportunity." If the airline fails to do this (for example by not rerouting at the earliest opportunity but waiting a bit longer), then I'd imagine that the passenger would be entitled to compensation from the airline (probably corresponding to what the passenger paid for rerouting at his own expense).

It is unclear what "under comparable transport conditions" means. If the other flight was "under comparable transport conditions" then the OP is maybe entitled to compensation for the new flight.

At the very least, the OP will have to pay for one of the flights: either the new one or the original one.Both Copenhagen and Belfast are on the €250 regime. However, it may be more expensive to reroute those passengers if there are fewer available seats on flights to Copenhagen.