Jira Cloud Migration to Data Center results to usernames being their AccountID rather than a normal

[Qusai Atoon]

Hello, 

we are migrating our Jira Cloud site to be a Data Center site. 

Problem:

After importing the backup file, and accessing Jira as the "sysadmin" user mentioned in the documentation, we found that all migrated users from cloud Jira have their usernames as their "AccountID". 

We tried to update the usernames in the DB after stopping Jira, but whenever we update a username for user we find that it shows that the user doesn't exist in Jira. 

The question is: 

How to update all the usernames in the cwd_user table in the DB without corrupting the users in Jira. 

(We are thinking of deriving the usernames from the users email where the usernames will be whatever comes before the '@' sign ex: qusia.atoon@gmail.com ===> username: qusai.atoon)