How can I bulk change user's login names

Below is the basic syntax of editing a single user with API.
How can I create and API file to modify this loginname to have OLD at the end awojtytOLD
Can I do that either with API or easier with a spreadsheet import?

All of our users that have a login with @Southernco.com at the end we want to change their loginname and email address to have OLD at the end (@Southerco.comOld)

Is there any way to do that with either spreadsheet import or API import???
Thanks

{

"users":

[

{

"uuu_user_loginname":"awojtyt","uuu_user_status":"1","uuu_user_company":"SoCo","uuu_user_dateformat":"0","uuu_user_timezone_pref":"50","uuu_user_firstname":"Agnieszka", "uuu_user_lastname":"Zurawska", "uuu_user_email":"awojtyt@southernco.com"