Changing a survey structure after it has been sent will modify it for all the recipients (future and present) of a survey.
Deleting the fields or forms of an already sent and completed survey will lead to the removal of the fields or forms and will cause data loss, even if the survey has been locked.
If you modify the survey package introduction text or otherwise modify the survey package settings, this will also be reflected in existing survey packages, with the exception of modification of the email text - this will only be reflected in newly created survey invitations.
If the automation engine is used for sending a survey, and if you have modified the email invitation text, since the existing survey invitations will contain the old text, you would need to archive those survey invitations and recreate them manually.
For Castor Connect, if downloaded locally, surveys will be completable in the form it was downloaded and submitted as such.