After a mailbox is moved from Microsoft Exchange Server 2007 or Exchange Server 2010 to Exchange Server 2013 or Exchange Server 2016, users who have Author permissions for a shared calendar of the mailbox cannot edit existing items that they created. Additionally, they receive the following error message:
Your changes cannot be saved because you don't have permission to modify some or all of the items in this folder. Do you want to save a copy of this item in the default folder for the item? Note
Users can edit items that were created after the mailbox was moved.