For bulk modifications, I would lean towards scripting if you are knowledgeable and comfortable at the level. If you are not comfortable with scripting, I would then utilize 3rd party tools like Managed Engine’s ADManager Plus which allows bulk modifications of user accounts. With the modification, you can assign the individual user to that mailbox, remove the positional group. Once that is all done, you can modify the mailbox settings as per default e-mail address or add a new PRIMARY SMTP e-mail with the new mail address format so no e-mails are dropped utilizing the old format.
You may additionally want to modify the recipients policy to the standard format you desire of email@example.com