app.open() details

Here's a Windows-based implementation of @pixxxelschubser's suggestion, which can change an existing document's units between inches, millimeters and centimeters. 

var units = "i";  // inches
// var units = "m";  // millimeters
// var units = "c";  // centimeters
var temp;
{
    interactWithDialog();
    app.executeMenuCommand("unitundoPref");
}
function interactWithDialog() {
    var contents = 
        'Set WshShell = WScript.CreateObject("WScript.Shell")\n' + 
        'WshShell.SendKeys "' + units + '"\n' + 
        'WshShell.SendKeys "{ENTER}"\n';
    temp = new File("~/Desktop/temp.vbs");
    temp.open("w");
    temp.write(contents);
    temp.close();
    temp.execute();
}
temp.remove();

IMHO (and unforturnately) the ruler units cannot be set for existing documents.

I have seen scripts that are supposed to make this possible, but they did not work for me. (For new documents, however, it is possible).

Existing open documents could be set (as a workaround) with a series of key combinations (preferences press [Ctrl]+[K] -> click on Units tab -> press [Tab] three times and then -> cursor up or down (several times). Maybe this will help you a little bit.

But maybe someone knows a script that works.

It's an OpenOptions() object you can create.