Objection 1. It would seem that a mortal sin can become venial. Because venial sin is equally distant from mortal, as mortal sin is from venial. But a venial sin can become mortal, as stated above (Article 5). Therefore also a mortal sin can become venial.
Objection 2. Further, venial and mortal sin are said to differ in this, that he who sins mortally loves a creature more than God, while he who sins venially loves the creature less than God. Now it may happen that a person in committing a sin generically mortal, loves a creature less than God; for instance, if anyone being ignorant that simple fornication is a mortal sin, and contrary to the love of God, commits the sin of fornication, yet so as to be ready, for the love of God, to refrain from that sin if he knew that by committing it he was acting counter to the love of God. Therefore his will be a venial sin; and accordingly a mortal sin can become venial.
Objection 3. Further, as stated above (5, Objection 3), good is more distant from evil, than venial from mortal sin. But an act which is evil in itself, can become good; thus to kill a man may be an act of justice, as when a judge condemns a thief to death. Much more therefore can a mortal sin become venial.
On the contrary, An eternal thing can never become temporal. But mortal sin deserves eternal punishment, whereas venial sin deserves temporal punishment. Therefore a mortal sin can never become venial.
I answer that, Venial and mortal differ as perfect and imperfect in the genus of sin, as stated above (1, ad 1). Now the imperfect can become perfect, by some sort of addition: and, consequently, a venial sin can become mortal, by the addition of some deformity pertaining to the genus of mortal sin, as when a man utters an idle word for the purpose of fornication. On the other hand, the perfect cannot become imperfect, by addition; and so a mortal sin cannot become venial, by the addition of a deformity pertaining to the genus of venial sin, for the sin is not diminished if a man commit fornication in order to utter an idle word; rather is it aggravated by the additional deformity.
Nevertheless a sin which is generically mortal, can become venial by reason of the imperfection of the act, because then it does not completely fulfil the conditions of a moral act, since it is not a deliberate, but a sudden act, as is evident from what we have said above (Article 2). This happens by a kind of subtraction, namely, of deliberate reason. And since a moral act takes its species from deliberate reason, the result is that by such a subtraction the species of the act is destroyed.
Reply to Objection 1. Venial differs from mortal as imperfect from perfect, even as a boy differs from a man. But the boy becomes a man and not vice versa. Hence the argument does not prove.
Reply to Objection 2. If the ignorance be such as to excuse sin altogether, as the ignorance of a madman or an imbecile, then he that commits fornication in a state of such ignorance, commits no sin either mortal or venial. But if the ignorance be not invincible, then the ignorance itself is a sin, and contains within itself the lack of the love of God, in so far as a man neglects to learn those things whereby he can safeguard himself in the love of God.
Reply to Objection 3. As Augustine says (Contra Mendacium vii), "those things which are evil in themselves, cannot be well done for any good end." Now murder is the slaying of the innocent, and this can nowise be well done. But, as Augustine states (De Lib. Arb. i, 4,5), the judge who sentences a thief to death, or the soldier who slays the enemy of the common weal, are not murderers.
Source: Summa Theologica (New Advent)