To exchange, change, or transform—swapping one thing for another. Paul uses it twice in Romans 1 to describe humanity's tragic exchanges: they traded God's glory for idols (1:25) and exchanged natural relations for unnatural ones (1:26). The verb carries commercial overtones—a deliberate transaction where something valuable is traded for something worthless. Classical usage included exchanging fates, assuming different natures (like birds), adopting new customs, or even dying ('exchange life for death'). The middle voice suggests active participation in the exchange.