The reason why it has to start again is probably because you are streaming through
http://" if your video file is streamed through "RTMP" then when you toggle between HD and Regular it keeps you right where you were in the movie or video. (this is using JW Player)
in the example below i am using Flash Media Server to stream a video using Real Time Mesaaging Protocol or RTMP.
here is an example of the script im using:
<script type='text/javascript' src='/movies/pelis/_player/swfobject.js'></script>
<div id='mediaspace' class="c1">This text will be replaced</div>
<script type='text/javascript'>
var so = new SWFObject('/movies/pelis/_player/player.swf','mp1','768','432','9','#000000');
so.addParam('allowfullscreen','true');
so.addParam('allowscriptaccess','always');
so.addParam('wmode','opaque');
so.addVariable('file','cienciaficcion-aliciaenelpaisdelasmaravillas.f4v');
so.addVariable('plugins','hd-1');
so.addVariable('hd.file', 'cienciaficcion-aliciaenelpaisdelasmaravillas-hd.f4v');
so.addVariable('autostart','true');
so.addVariable('stretching','exactfit');
so.addVariable('streamer','rtmp://your-server.com/vod');
so.write('mediaspace');
</script>