In addition to Mike response, the Audit folder can be located in a network drive different than the one setup for the Repository. 

To known the exact location go to Laserfiche Administration console, go to Auditing node, then check the Audit file location section to find out the exact path for the Audit folder. Also keep in mind that the Rollover Directory might be in a different path as well, this is the folder that contains the rollover log audit files. There is no need to merge two different database, if you install Audit Trail version 10 on top of Audit Trail 8, then the current Audit Trail database for version 8 will be updated , this means the database schema will be updated. Depending on how you have setup the Audit Trail range dates or days you will need to either add another Date Range to load more data from the log files.  If you have it set to load the last 30 days then the process should be automatically. 

Thanks for the answers!  A follow up I got from our client is how can we point the current version of Audit Trail back towards the old data?

When you upgraded to v10, did you manually change the location of the Audit Logs?  If not, both the old and the new data should be available in Audit Trail.  You just need to make sure that the Date Range (in the Audit Trail Config page) is set back far enough to include the v8 data (see screenshot).