Thanks to Workflow in Microsoft Dynamics CRM 4.0, you can now setup an Account-to-Lead, Contact-to-Lead or Opportunity-to-Lead function.

It's pretty simple to do.  Just create the workflow for the entity that you want to use to create the new Lead.  You'll create a new record (of type Lead) and populate the lead fields with items from the Account, Contact and/or Opportunity.  I recommend attaching the workflow to either the Contact or Opportunity b/c the Account really won't have all the fields that you want to use in order to re-create the Opportunity.

If you want to get fancy, you can test to see if there is an Originating Lead.  If there is, you can just reactivate the lead and send an alert to the Lead Manager (the owner of the Lead record) to please try to re-connect with the lead again.

Train your users that, once the lead has been re-qualified, when they convert it, they should ONLY select Opportunity (not Account or Contact) - and they can then re-attach the new opportunity to the correct account.