You can edit Guided Reading Mode settings after creating an assignment. Open the Edit Assignment settings on your existing assignment by clicking the pencil icon ("Edit Assignment" on mobile) for the assignment on the Assignments page (or the assignment's Dashboard).
From there, if the assignment has guided reading Mode questions, you will be see an "Enable Guided Reading Mode?" question followed by a "Yes/No" toggle. To enable Guided Reading Mode, set the toggle to Yes and choose whether to assign to all or specific students. To disable Guided Reading Mode, set the toggle to no. Click Save and instruct students who are currently viewing the assignment to refresh the page to see guiding questions appear or disappear, respectively.
If you enable Guided Reading mode for a student who has already submitted the assignment, you will not see any Guided Reading Mode data for that student unless you unsubmit them, allowing them to access the guiding questions. If you disabled Guided Reading Mode for a student who had already submitted the assignment, their data is still saved, but hidden from your Guided Reading Mode data report. If you reenable Guided Reading Mode for that student, their prior data will reappear.