It is possible to bulk update the Event Level for attendees using the Apex Data Loader. Please note that the Apex Data Loader is a Salesforce tool, so you'll need to contact Salesforce Support for any assistance with this tool.
  1. Run the Event Attendee report.
  2. Add filters to pull attendees for the specific event and level.
  3. Add the Event Attendee ID field to the report.
  4. Run the report and export the results as a CSV file.
  5. Add the desired Event Level ID to your file; this will be the new level for your selected event attendees. 
  6. In the Apex Data Loader, run the Update operation.
  7. Mark the checkbox box at the top to "Show all Salesforce objects".
  8. Choose Event Attendee as the object and select the CSV file you just created as the target.
  9. Map the ID field to the Event Attendee ID column in your file.
  10. Run the operation to update the selected attendees.
For more information on using the Apex Data Loader, please see How to update info in LCRM using the Data Loader (BB745020).