Exchanging two creatures’ text boxes means that each one loses its rules text and replaces it with the rules text the other one had. In Un- games, this also means the text boxes themselves are exchanged, including the flavor text and watermarks in those text boxes. In both cases, this is a text-changing effect.

Notably, ability stickers (and other granted abilities) aren’t part of the rules text that get exchanged. Any ability stickers on either of the two permanents continue to apply to the permanent they are on.

If Exchange of Words leaves the battlefield before its enters-the-battlefield ability resolves, the text boxes are never exchanged. Neither creature is ever affected. The same is true if either of the two creatures becomes an illegal target before the enters-the-battlefield ability resolved.

Once the exchange has happened, either of the two creatures leaving the battlefield has no effect on the other creature’s text box. The exchange will only end once Exchange of Words is no longer on the battlefield. Similarly, further changes to either creature’s text box won’t change the other’s text box.

If Exchange of Words has lost its own abilities (or perhaps its text box), the effect will still only end once it is no longer on the battlefield.