Short answer: Since they are justified in Christ, sin won't exclude them from salvation but they should still follow the law out of love for God and repent for their transgressions out of conscience.

Matthew 5,17: “Do not think that I have come to abolish the law or the prophets; I have come not to abolish but to fulfill." (referencing prophesied salvation)

Matthew 22,36-40: “Teacher, which commandment in the law is the greatest?” He said to him, “‘You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.’ This is the greatest and first commandment. And a second is like it: ‘You shall love your neighbor as yourself.’ On these two commandments hang all the law and the prophets.”

However, the matter of how the old law should be applied by Christians is as old as the movement itself. A big argument in the early church was whether adult converts were required to be circumcised. Paul, evangelizing in the gentile (uncircumcised) world, definitely falls on the lenient side in these regards. Since his commentaries are featured in the Bible itself, thats how most modern Christians interpret the old laws.

1 Corinthian 6,12: “All things are lawful for me,” but not all things are beneficial. “All things are lawful for me,” but I will not be dominated by anything.