Metallassō (μεταλλάσσω, G3235) means to exchange, to trade one thing for another, to substitute, to change completely. From meta (change/after) + allassō (to change/alter). Appears only in Romans 1:25,26 — 'They exchanged (metallaxan) the truth of God for a lie' and 'Even their women exchanged (metallaxan) natural sexual relations for unnatural ones.' The word describes a deliberate, total trade — giving up one thing completely for another.
Romans 1:25 presents the fundamental human problem as an exchange economy of idolatry: humanity traded (metallassō) the truth for a lie, the Creator for the creature. This exchange did not happen once in Eden and then stop — it is the ongoing transaction of fallen human hearts. Paul's point is that sexual immorality is not the cause but the consequence of this prior exchange; God 'gave them over' (Romans 1:24,26,28) to the outworking of their core idolatry. The theology of exchange runs throughout Scripture in two directions: humanity exchanges glory for shame (Jeremiah 2:11: 'My people have exchanged their glorious God for worthless idols'); God exchanges our shame for glory in Christ (2 Corinthians 5:21: 'God made him who had no sin to be sin for us, so that in him we might become the righteousness of God'). The metallassō of Romans 1 is answered by the substitutionary exchange of the cross.