I don't know why the airlines shank you like that when you want to change your flight and don't know if it's something exclusive to US based airlines. I wanted to do the same thing once. Change my departure date a week or so. The airline (US based carrier) wanted almost $1000.00 to do so. Yikes! Needless to say, I didn't do it.
When I was working overseas, twice I needed to change departure dates by a few days after I had booked flights. Emirates changed it for free, and when I had a flight changed with Qatar Airlines it cost me like $40.00 (US).
Personally I wouldn't give the airlines the satisfaction of getting my hard earned $600.00. The whole going to Finisterre thing is pretty cool, but a bit overrated (I'm sure I'll get criticized for saying that
) and I think it being in that movie "The Way" has made seem a Camino must do. Concentrate on getting to Santiago. If you like it, maybe do it again someday all the way from SJPdP to the coast.
cheers