[zlatko]

Сделал так :

<?xml version="1.0" encoding="utf-8"?>
<mx:TextInput xmlns:mx="http://www.adobe.com/2006/mxml" change="onChange(event)"> 
  <mx:Script>
    <![CDATA[
      private function onChange(event:Event):void
      {
        text = text.replace(/A/g,"B");
      }
    ]]>
  </mx:Script>
</mx:TextInput>