One method for alternative display is making use of the html <OBJECT> tag, which allows the specification of alternative content. Read up on it in this part of the html specs, or follow this example from there.

<P><!-- First, try the Python applet -->
<OBJECT title="The Earth as seen from space" 
     classid="http://www.observer.mars/TheEarth.py">
         <!-- Else, try the MPEG video -->
  <OBJECT data="TheEarth.mpeg" type="application/mpeg">
           <!-- Else, try the GIF image -->
    <OBJECT data="TheEarth.gif" type="image/gif">
             <!-- Else render the text -->
      The <STRONG>Earth</STRONG> as seen from space.
    </OBJECT>
  </OBJECT>
</OBJECT>
[download]

I'm not sure if it will work with multiple versions of flash, but it may be worth a try.