It means that Barcelona has changed so much that if someone who had lived there a long time ago came back to visit, he would not recognize it. For example, there might be many new high buildings, replacing or hiding the older buildings. There might be many new roads, and the city might be many times larger than before.
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khoffI believe it would be more common to say, "Barcelona has changed beyond all recognition." I've never heard "changed out of all recognition" before.Well, Khoff, I haven't either. But it was a quote from a book, and I offered an educated opinion!