I've tried to change the username in jira from firstname to firstname.lastname.
I've made a backup of the system, change all the entries in entities.xml
from firstname to firstname.lastname with notepad++ and saved the file to the backup .zip.
as i imported the changed backup, the system setup of jira begun from scratch, with all the basic informations.
do you have a idea, what went wrong? or is it newly possible to change the username within jira, and all tickets are still correct with the assignee firstname.lastname?