Our programs have many repeat volunteers as well as many new volunteers. Therefore, every time we do an import we have hundreds of exceptions. The only way to get a list of those exceptions with an identifier that can be used to actually check those records is to manually enter the Constituent ID. If the initial import allowed us to create a query of those exceptions (instead of an import file), we could just use the query to output information for each record so we can check to see if we have new information for an existing constituent.

I'm still somewhat of a newbie, and may be missing something, but this is the best explanation I have found on how to do this: https://community.blackbaud.com/blogs/10/754  and that seems like an unnecessarily laborious process.