All scripture is valid. Each has its time and place. Do you consider the prophecy in Is 7:14 about a virgin with son to be invalid? No it has been fulfilled but not made invalid.
St. Paul says that if we were baptized into Christ we also died with him and we are now dead to the law. Most scholars take the law to mean the Mosaic law. St. Paul says that the law was unable to save us because we transgressed it. Therefore, all the law did was point out our sins but it couldn't save us from those sins. It is only Jesus that can save us from our sins since he died as a ransom to take away those sins. The blood of bulls and goats under the old covenant was never enough to redeem someone from their sins against God. It is only an eternal person who could pay for an eternal offense. And, as Fr. Mitch Pacwa says to offend God is an eternal offense because God is eternal.
Having been baptized into Christ we died to the law so that we are no longer under law but now under the Spirit of Christ. This means that the Mosaic laws under the old covenant including even the 10 commandments are no longer binding on Christians, because these were under the old covenant which no longer applies to someone who has been 'buried with Christ'.
Now, someone might ask then why do we keep the 10 commandments if we are no longer under law? The catechism states that we do not keep the 10 commandments because they were given by Moses but because they are consistent with God's Eternal law. In other words, God's moral laws can never be abolished. Thus, we don't keep the 10 commandments because they are part of the old covenant, but rather because they are part of the new covenant because they are eternal laws. Jesus himself on the sermon on the mount validates the moral commandments and sums them up in the phrase to love the Lord your God with all your heart and to love your neighbor as yourself. In other words, for instance, the ban on killing your neighbor can never be changed regardless of what covenant you happen to be living under because it is a moral law that a God of love and mercy consistently requires in his people. Just because your sins are forgiven in Christ Jesus doesn't mean you can go kill someone.
But, there are other laws, commonly called 'ceremonial' laws like circumcision and eating pork that are not moral in nature and are therefore not eternal and therefore not binding on Christians who have 'died with Christ'. This is why for instance Christians don't practice circumcision or can eat pork. Or why we don't practice the Sabbath on Saturday for instance because which day and how it was practiced was all part of the ceremonial law system of the old covenant. The spirit of having a day of rest and honoring God is practiced by the Church on Sunday. It is not a moral issue on which day you take your day of rest. Sunday became the day of rest for Christians because that is the day Jesus rose from the dead.