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In order to edit set of Scheduled Issues just select them and press the "Edit Selected" button:
Use the drop-down menu cloaked under the "Projects" (1) to access a desired one and then go to the below located "Scheduled Issues" (2).
"Edit Selected" (4) button becomes active when you tick (3) particular scheduled issue(s) from the list above that button.
You will be redirected to the page where you can amend all necessary settings which include changes in fields available to particular issue type under a proper project.
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