You are able to integrate the content of the DLC  with a simple click of a button into an existing Story campaign without having to start over. Alternatively, all new Story campaigns, with JOTL activated, will have all new mercenaries available from the start and the new scenario content available after core mission 2.

Note - DLC enabled save games cannot be reverted to non-DLC games.

If you play in the Guildmaster mode, once converting the save or starting a new game the 4 mercenaries will be immediately available and the items will be added to various treasure tables.