A: No. If a customer completes the same form again with new information, this doesn't overwrite the information they added before. Fields that already have a value on the contact or company keep that value.
Fields that are empty on the contact or company, including custom fields, are filled in from the new submission. If the contact is inactive, the submission makes it active again.
Which contact and company the submission updates is described in How form submissions are matched in Upsales.