I just found one of these when booking BCN-JFK-BCN for June. As the flights are five months away (and therefore the possibility of needing a change is more likely), I checked the fare rules while I had the booking on hold. As expected, it says €150 change fee.
But then I noticed that the return fare rules had a different code from the outbound (O instead of N) and found in the Penalties section: "CHANGES ANYTIME: CHANGES NOT PERMITTED". And because the most restrictive fare rules apply to the entire itinerary, this probably means that the outbound is not changeable either.
So I called the EXP desk to see if I could up-fare the return segment to the same as the outbound, but the agent insisted that the return IS changeable. She confirmed that the fare rule - OLN8D0Z1 - was the same one that I was looking at.
So I asked her to put a note in the record that either journey CAN be changed for a €150 fee, which she willingly did.
However, she could not email that to me, so there is no way that I can get that text in writing.
Just a warning to others that there are still some NON-CHANGEABLE fares out there, and there seems to be some inconsistency between the rules that we see, and the rules that the agents see.