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I would like to inform you that you will not lose submission data if you add new fields to your form. However, if you would like to make modifications to a form without editing the original, then I would suggest you clone the original form instead.
By cloning the form, you will be able to have the original form as is, and you can simply add the questions into the newly cloned version of the form. You can follow this guide on How to clone an existing form from your account.
If you need further assistance with cloning, feel free to let us know.