Fixing leading by script

@Rogerio5C09 -- Look no further than the two lines that @Robert at ID-Tasker suggested, repeated here for convenience:

doc.stories.everyItem().leading = '14.4pt';
doc.stories.everyItem().spaceBefore = '2mm';

 This is all you need. But for interests' sake, apart from the comments that you didn't call your function, the function is inefficient because it resets the find/change preferences at every iteration. Get these settings out of the for-loop. o your script would look as follows:

function fixLeading(){
  var doc = app.activeDocument;
  var leadingPoints = ["17pt","18pt","19pt","20pt"];
  app.findTextPreferences = app.changeTextPreferences = null;
  app.changeTextPreferences.leading = "14.4pt";
  app.changeTextPreferences.spaceBefore = "2mm";
  for (var i = 0; i < leadingPoints.length; i++) {
    app.findTextPreferences.leading = leadingPoints[i];
    doc.changeText();
  }
}

fixLeading();

By the way, why do you look for those four point sizes? Are they the leading values in your document? The two-liner doesn't care about existing leading, it simply sets all text to 14.4pt leading.