You'd need to manually add fields to the form if you want to update it via GUI. If you want you add additional fields to the draft version of the form (only the approved state is used on pages where it is placed), and approve it once you're ready, but this is slightly risky as if any other user approves the form before the intended date, you'd not be able to roll-back automatically. I'd recommend you add fields you want for now, approve it, and once you're near the next launch (i.e., near to roll-out additional fields on the form), update the form to have additional fields, and then approve the updated Draft version. Note: you'd need to approve the LPs where the form has been added via the form element (no need to do so if the form has been added using the embed code though).
