It is easier and more secure to parse the employee data file set by set, search for the username in a directory server (not an LDIF file) and do updates on the fly:

If you export a file, modify it and import it again, you waste time in which other people or jobs may have changed the data (if no other jobs are yet manipulating the data, it may happen in future). So keep the time a modify takes as short as possible => do online updates. And for importing the data again, a script that does updates instead of delete/add is necessary. so why not enhancing this script with the search?