How to change the kind of a Keychain item?
You can't directly change the kind of a Keychain item because item details use different schemas.

  generic        internet
  ═══════        ════════
  account ────── account
  creator ────── creator
  type    ────── type
  kind    ───┐  ★domain
 ★value      └── kind
  comment ────── comment
  label   ────── label
 ★service       ★path
  password───┐   port
             │  ★protocol
             │  ★server
             │  ★authenticationType
             └── password


You'll need to read out the information for the item you want to change, then create a new item with the new values where applicable.