@priyanka90 take a snap shot of all the user's role assignments via saved search then use the result of the save search later on to mass update the employees after the email change. You have to remember that email address is main key in the Netsuite access table, when you change the email you basically wiping off all the roles assigned to older email - this will be a brand new access table in the database. Come to think of it, I could create a saved search to generate a complete user table - internal id/external id, email, roles, access and password(you assign) then use the saved search data to create the new upload data (Transform part of ETL) and update. Try a small batch first.

@Sam-I-Am I tried what you have suggested. But is it possible to update email id without giving password in CSV file. If we do so all users wont be able to login as their password will be modified via CSV. And we wont be able to communicate them for same. Any other suggestion?