Lead Nurturing - Qualification Level

My customer is working on lead nurturing and have the following criteria:

1. Use campaign management to send out communication to customers with an existing lead.
   1. Use an email and a survey to communicate to the customer.
   2. Use a target group on the campaign with customers with leads of a certain status.
2. The qualification of the lead would be updated based on the survey results total points.
3. The customer would not have any log-in credentials.
4. The customer could have multiple leads in the system but only one should be updated.

When the survey is completed we would like the qualification level field be updated in the CRM system.  I realize a lead can be created from a survey but can the lead be updated from a survey?  How is the lead identified in the survey if the target group doesn’t hold this information?  How does the survey pass enough information for the lead to be identified and therefore updated.

Any help would be appreciated.