Only thing I can suggest is that you Quote your javascript external call ...
Code:
<script type="text/javascript" src=$vboptions[bburl]/clientscript/vbulletin_chgclass.js></script>
Should be
Code:
<script type="text/javascript" src="$vboptions[bburl]/clientscript/vbulletin_chgclass.js"></script>
for validity purposes - might also cause a breakage somewhere as neither this nor the other script has broken on either Firefox or IE6 or Opera for that matter ...
EDIT: I've updeated the original instructions to include the quotes ... and I'll have to upgrade to 3.0.1 at some stage to see if that breaks mine ...