In Jira Advanced Roadmaps, I want to change the parent ticket to a type that's at a lower level in the hierarchy. The child tickets are already at that level. How can I make this change, and also move the child tickets down a level?
The hierarchy in Plans (Advanced Roadmaps) is fixed. So the only options you have here is to:
either change your hierarchy configuration and bring both issue types down one level. That is only possible if these 2 issue types are defined above the epic level (epic, story and sub-task levels are fixed and cannot be changed). That will bring everything down 1 level, but be aware that this change would apply to all plans you have in your Jira system.
or move all issues to a different issue type that is at the - for you - right level in the hierarchy. That is theoretically possible with bulk change > move, but may turn out to be complex, depending on what issue types you currently have.
