1. In the speech editor, click the "Add" button. The name of the speech goes under the "Remove" button. In the Random 1 field choose the node to assign to this speech. And under that enter an event name like "NPCspoke1".
2. For the trigger condition use "Speech Occured". Here you choose the speech event from above - "NPCspoke1".
3. For the trigger action use "Change Player". Pick the tag name you used for the entity from the list for the Unit. Then you want to change him to Player 1.