I need to sort an already sorted binary file, what's the best way/algorithm to use?

I've got the binary file containing records for a personnel system. It's already sorted based on their employee numbers. But I need to sort them based on other fields (e.g their last name). Keeping in mind the slow I/O of files, and that there's over 100,000 records in the file, what would be the best way to sort this file? I'm using C#.NET 2008.