As soon as I wrote the previous post, I went to the reception and changed my reservation from five to three nights. Turns out I was just in time – the deadline for cancellation was at 22:00, and my change went through at 21:56. This means I won’t be spending the Easter morning in Madrid. :)
Now I just need to find a decent hostel in Barcelona. “Decent” means “close to the sea and away from the crowd”.