How to Mass Update One Segment Value in All Employees' Default Expense Accounts?

Summary

How to change one segment of the default expense account for all employees?

Content

We are seeking to change one segment of the default expense account for all employees (about 1200.) This particular segment would be changing from a default of '0000' to a default of '0999' but the other segments in the employees' default expense accounts should be left alone.
 

Additionally, we would be seeking to change the work relationship start date to one default date for all employees. Could this be done at the same time in the same way, or would this be handled separately?