Jquery: Possible to dynamically change source of Autocomplete widget?

Try actually changing the source option of the autocomplete on the change event.

$(function () {
    var select = $( "#selectType" ),
        options = select.find( "option" ),
        address = $( "#address" );

    var selectType = options.filter( ":selected" ).attr( "value" );
    address.autocomplete({
        source: "ajaxSearchForClientAddress.php?selectType=" + selectType,
        minLength: 3
    });

    select.change(function () {
        selectType = options.filter( ":selected" ).attr( "value" );
        address.autocomplete( "option", "source", "ajaxSearchForClientAddress.php?selectType=" + selectType );
    });
});