Can I modify/extend a production web service without affecting existing clients?

I am currently calling a web service that returns a service defined class which I am interpreting in my application. I’m considering asking the vendor of this web service to add a property to this class which will make my life as well as their other clients lives a lot easier. To be clear I’m not asking them to modify existing behaviour or properties, so this would extend existing functionality.

My question is, if they add this property to the class, will it adversely affect existing clients’ applications?

Image URL error catching problem

I am trying to see if a URL exists or not, and if it does not exist reset the variable containing the URL to an error page/image. However my attempt below sets every instance of PIC_URL to noimage.gif, regardless of if it exists and is accessible or not. What am I missing?

if (@fopen($_REQUEST[$PIC_URL],"r")) 
$status = "1";
else  if (!(@fopen($_REQUEST[$PIC_URL],"r"))

$status = "2"; 

if ( $status == "2" ) $PIC_URL = "";