Hey Gregory, Perhaps you're making this a little overly complicated. Why not just use standard progressive profiling? If you know someone already, they won't be shown the early stage questions of name & company. They will only see the fields they haven't yet filled out, the late stage questions.
A second option is to as you suggest, use dynamic content. Then you would have to create a segment, known users, and dynamically segment on whether they are known, if they are known then show them a 2nd form.
For a non-tech guy, these are the two ways to go. You don't want to be writing custom javascript to query the database to determine which form to display. Either use progressive profiling or dynamic content.