property oldIndex

This property returns the index of the old selected item. This index is a zero-based row index. It is NOT a box index, so don't pass it to methods like getChildAt. Methods like getBoxAtRow on select box use this row index.

<comboBox on:select="onSelect();"/>

function onSelect()
{
    var eventObj = shell.currentEvent;
    if (eventObj)
    {
        var oldIndex = eventObj.oldIndex;
        var selectBox = eventObj.targetBox;
        if (oldIndex && selectBox)
        {
            var previousSelectedItem = selectBox.getBoxAtRow(oldIndex);
        }
    }
}