You'll want to probably do the equivalent of an AJAX call via Flex. I don't have it in front of me but I think you can use the HttpRequest object.
You'll basically have your object set up to retrieve the xml with a result handler on it. Whenever you need to call it, do so:
and then you'll get your result via your handler:
private function myResultHandler(event:ResultEvent):void
//do your handling stuff here....
obviously if you using things like ArrayCollections for drop downs and such, life will be a lot easier for you.
Hope that was at all helpful,