I don’t think adding items to most choice-based fields should affect saved instances of the form when they revisit the site to complete. The one exception to this would be checkboxes. According to documentation for those:
Important: Once the form has entries the choices should not be changed. Adding, removing, re-ordering, or renaming checkbox choices will result in the input ids associated with the choices changing which could break the connection to the choices already stored in the existing entries and prevent them being exported.
Also note that if you change the URL on which the form is embedded, then the users will not be able to directly return to their saved instance.