Migrate identity (ECID)

Every visitor Adobe has ever seen carries an identity, the ECID, and your site has been storing it in a cookie for years. The risk in this migration isn't losing data; it's losing continuity. If the Web SDK starts minting fresh IDs, every returning visitor looks brand-new overnight. One checkbox prevents this: it tells the Web SDK to read the identity already in the visitor's cookie and keep using it, so the person who bought from you last week is still the same person this week. It is the smallest action in the whole migration with the largest downside if missed.