Yes, you can edit Guided Reading Mode (GRM) 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 from the assignment's Dashboard page).
From there, if the assignment has GRM questions, you will see an "Enable Guided Reading Mode?" question followed by a "Yes/No" toggle. To enable this feature, set the toggle to Yes and choose whether to assign to all or specific students. To disable this feature, set the toggle to no. Click Save and instruct students who are currently viewing the assignment to refresh the page in order 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 GRM data for that student unless you unsubmit them, which will then allow them to access the guiding questions.
If you disabled Guided Reading Mode for a student who had already submitted the assignment, their data will still be saved, but hidden from your GRM data report. If you reenable Guided Reading Mode for that student, their prior data will reappear.