/USE_EXISTING
       /NOUSE_EXISTING (default)

    By default, a migration to a popstore or msgstore account will
    fail when there already is a pre-existing account with the same
    name as the requested destination account name (by default, the
    name of the source store account being migrated). To override
    this behavior, specify /USE_EXISTING. However, if the pre-
    existing account has the msgstore/popstore MIGRATED flag set,
    the migration will fail unless /FORCE_MIGRATE is also specified.
    Note that when migrating to a pre-existing account, the password
    for the account is left unchanged and migrated messages are added
    to those already stored for the account.