frame within current form & use id for Hyperlink target

You mean like:

<form action="someaction" method="post">
    <iframe src="somesource.html" width="x" height="y" />
</form>

<a href="#" onclick="swap_source()">Change I frame source</a>

I would think there's a method in Javascript to change the source attribute of an iframe, but I'm not sure I 100% understand your question.