Hi Vasilis,
To allow existing registrants to add the newly introduced optional item, start by ensuring that the item is visible and available to the appropriate registration types within the registration path. Then, send a targeted communication to the registrants who completed their registration before the item was added. In your email, include a "Modify Registration" button that allows them to access and update their registration. Be sure to clearly mention that a new item has been added and they can revisit their registration to include it if they wish.
Please contact me if you need any assistance with your event in Cvent.
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Amarnath Annamalai
Freelance Cvent Specialist
www.cvntexpert.cominfo@cvntexpert.com------------------------------