You can't reschedule this event

Problem

Unable to modify users' events while signed in with the super admin account

Environment

Calendar web IU

Solution

As the super admin you can subscribe to any calendar within your organization but when trying to make changes to users' events, make sure that the user's calendar is the only one selected at the bottom left hand side (subscribed Calendars section) 

Cause

You can't change users' events from the calendar of a guest.