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After you the plan is created a plan it’s still possible to edit its details. You can change its update the name and duration. Bare in mind, but remember that tasks that will that changing the start or end date may cause some tasks to fall outside of the new range timeline. Such tasks will be either removed or shortened to fit the new range.
To edit the plan you need to be either a Jira admin or a creator of this plan.
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Here you can change the plan name.
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The main project can not that was selected while creating a plan cannot be changed after you created a plan. You can still, however, add issues from other projects.
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