Updating Records

[crazybake]

What are the best practices for updating records?

For example, every three months I receive an Excel file containing data (name, event ID, location time, etc.). These Excel files are set up identically, so it makes it easy to import through a script. I use this information to budget, staff and schedule. However, as each new Excel file is sent to me, the previous Excel file is updated with data (such as number of attendees for each event that occurred). I created new fields to accommodate this information and want to 'add' the new information to the existing records automatically (there are several hundred events every three months and I would hate to update manually). What is the best way to re-import just the new data into the fields - and find/match so that the correct attendees data finds/matches the existing event records?