Think of it this way:
Once you are retreating units out of combat, they may not perform any more action(s) that turn. Their only action is to move 1 step to an adjacent space (planes excepted).
Sankt is correct. The SUB may still chose to leave the SZ in COMBAT movement phase of their upcoming turn. However, if the SUB does not combat mvoe out, then Comabt will occur with the new fleet.