Well, the world that have changed the most, in my opinion, were V, VI, and X.
The MOST changed was VI. It was the only game that ended with a destroyed world. Nothing was rebuilt or anything (of course, they probably did after many years). And of course V had a merging of two worlds. So an obvious major change there.
The only thing I saw that was altered in VII was Midgar. Nothing on a wide scale really. (Note I did not play any of the VII compilation games, though I did see Advent Children if that makes any difference on opinion)
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