I want to change all booked appointments from In-Person to Virtual (or vice versa). How can I do this?

If your meeting format changes for the entire day or a specific time block, the most efficient way to perform a "bulk transition" is by using the Slot Updater ; you don't have to edit individual registrations one by one. 
 In the Admin UI : 
 
 Open the event. 
 On the Event Management page, select the date on the Calendar where you are going to update slots. 
 Click Slots / Update Slots . 
 In the Slot Updater pop-up: 
 
 Set the desired time range. 
 Choose the new Appointment type from the drop-down. 
 Click OK. 
 
 
 A confirmation message will appear, informing you how many existing appointments will be updated. 
 Click OK to proceed. All affected appointments will be updated to the new type.