See https://developer.yahoo.com/fantasysports/guide/ for details

transaction_replace_player.xml

<fantasy_content>
  <transaction>
    <type>add/drop</type>
    <players>
      <player>
        <player_key>{player_key}</player_key>
        <transaction_data>
          <type>add</type>
          <destination_team_key>{team_key}</destination_team_key>
        </transaction_data>
      </player>
      <player>
        <player_key>{player_key}</player_key>
        <transaction_data>
          <type>drop</type>
          <source_team_key>{team_key}</source_team_key>
        </transaction_data>
      </player>
    </players>
  </transaction>
</fantasy_content>