Find/Change random scripting

I don't know what sample you were thinking of, but it's quite likely not meant to change items at random. That's a rare thing to ask Mind explaining why?

In any case, the following script (CS3 or newer!) does the trick. The findText function is fed with the correct parameters: the text to be found and case sensitive on, so it will not find 'Aa' or 'aA' or 'AA'. Since the findText function returns an array (rather than 'a next item', one by one), we can loop over the found items and change either its first character or its second. We do that simply by setting either character #0 or character #1 of its contents to the a-ring, leaving the other character alone. The backward loop is to prevent messing up the document, should any of the code change the text.

The math construction returns a random value between 0 and 2 inclusive, where 0 changes the first character, 1 the 2nd, and 2 does nothing (left out for brevity).

app.findChangeTextOptions.caseSensitive = true;
app.findTextPreferences = NothingEnum.nothing;
app.findTextPreferences.findWhat = "aa";

foundItems = app.activeDocument.findText();
for (var i=foundItems.length-1; i>=0; i--)
{
switch (Math.floor(Math.random()*3))
{
  case 0: foundItems.contents[0] = 'å'; break;
  case 1: foundItems.contents[1] = 'å'; break;
}
}

Note: untested!