Process Multiple files by group the files and spanning a thread

I am looking for a suggestion on design approach for processing multiple files(around 100, 500KB file each).

Basically need to update the old ID with new ID and corresponding update the new ID in dependent files(which has a reference of old ID).My thoughts were

1. Group the files based on dependency and span a new thread to process the files for ID replacement.

2. Having StreamReader against the original file and a StreamWriter against the destination file. Then, loop over all the rows returned by the StreamReader, use .Replace on each row, and write it to the StreamWriter.

Any better approach to achieve better performance.

thanks AR