The age of the defeated argument has no bearing on the fact that it was erroneous in the first place. The protection from error is subjective, not objective. If you do what the Chruch says, you are not at fault, even if the promulgation of the old doctrine was pious nonsense and possibly cravenly cynical (see indulgences). Back to marriage, if one is a victim in a marriage of abuse, alcoholism, rape, abandonment, etc., then one does not commit adultery by marrying again.