Finally! Since v6.4.4 (Server & Data Center) it is possible for Jira Administrators and users with Administer Project permission to change Creator in bulk operation.
Tip: consider creating a dedicated “The Scheduler User” as creator and reporter to make sure those templates are robust and issue creation notifications are sent.
For The Scheduler < v6.4.4
There is no straight forward option bulk change the creator of scheduled issues, but it is possible.
To bulk change creator you would need to export all of the scheduled issues, make a copy of the xport file, modify the export file to change the creator (and assignee), delete all invalid issues, import scheduled issues.
Downside is that with import option you would lose the the history of the executions of Scheduled Issues (it will be visible in upcoming version 5.1.2)
Here is detailed instruction what you can do:
Base preview of the Scheduled Issues table in project: Project Management, by lppl - project lead and project administrator for this particular project:
Same table with deactivated user "lp00"
Export all Scheduled Issues from this particular project (make sure you also export invalid issues) - this needs to be done by Jira Administrator (admin in my case)
Bulk deleting all Invalid Scheduled Issues:
Create a copy of Export file, and edit export file by changing the creator: "<createdBy>lp00</createdBy>" to desired user in my case lppl: "<createdBy>lppl</createdBy>" - it needs to be user with access to this project and with access granted through the access manager, this user should have at least permissions same as the deactivated user.
Import edited file
Table preview from project lead (lppl) perspective. as you can see now user have access to all of the issues and can edit them manually.
(But there are still some invalid issues.)
Inactive user as assignee
Invalid sub-task (in this case also invalid assignee)
Make a copy of the recently upload .xml file and make new changes, search and replace assignee field (make sure to copy pair of parameter and value, white spaces also counts)
Changing the sub-task assignee (different indentation - it wasnt found in previous search)
Delete invalid issues from the particular project (could be don as 9th step)
Import newest .xml file with changed assignee field.
Scheduled Issue table preview from the perspective of Project Lead - lppl.
As you might notice there are some warnings next to Scheduled Issues, those are informing you that the reporter field is empty and it will be filled with the creator of the issue.