This is covered in the "Changing fields & getting the changed data in your app" section of the custom license fields article: https://wyday.com/limelm/help/license-features/#change
Short answer: if the customer activated offline originally, have the customer activate offline again (generate a new activation request, have them send it to you, and send them back the new offline activation response) and it will include the newest custom license fields.
Or just force the customer do only do online activation and remove this end-user's self-created problem.