Hello @JonathanP75 ,
The only way to really do what you're describing would be to make a certain registration form field visible after a registrant has already submitted their info. However, you'd likely also want to make sure registration is closed, otherwise other registrants would be able to see and submit info accordingly.
In all honesty, I'd recommend waiting until registration is complete for the event - whether from you closing early, all registration spots filling up, or from the event itself completing, then exporting the registrants list. From there, you can manually update their information via your preferred spreadsheet program you use for viewing and editing CSV files.
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