If there are Units present at the Battlefield the Combat is taking place at, but have the opposite designation of their controller, they lose that designation, and gain the same designation as their controller now
§ 323.2.b Core Rules PDF (officieel)
§ 323 When a Cleanup occurs, the following Tasks become Outstanding in the order described:
§ 323.2 2. Assign or Remove the Attacker or Defender designation from Units as needed if there is a Combat in progress
