Plot Gods make it so Sinestro and the Yellow Lanterns get exiled in 616 Marvel. Meanwhile, all Heralds of Galactus get sent to current DC universe.
If there's some question of the Power Rings functioning properly in 616, Plot Gods grant that they do. Same with Heralds and Power Cosmic, if that's an issue.
Comment on any developments you think are interesting. For example, what happens when the human Yellow Lanterns drop by 616-Earth (I think Jonathan Crane is one, possibly others). What happens to the cosmic balance of power in either universe?
Does the Parallax entity necessarily come through with the rings? What happens if the Yellow Lanterns summon or recreate him?
If you want to concentrate your answer on only a few characters, like Silver Surfer flying around the DC universe, go ahead and do that. Choose whatever aspects of the scenario grab you, if any.
Bonus question: They fight. All Yellow Lanterns vs all Heralds of Galactus. Outer space battle, arena rules.