Challenge: How Could a Sin in the OT Not Be a Sin in the NT?

A few months back, one of our commenters asked for help answering this question, so I've made it this week's challenge:

If God is absolutely moral, because morality is absolute, and if the
nature of “right” and "wrong" surpasses space, time, and existence, and
if it is as much a fundamental property of reality as math, then why
were some things a sin in the Old Testament but not a sin in the New
Testament?

So are right and wrong absolute, or aren't they? How do you explain the change in laws? Let's hear what you have to say in the comments below, and then Brett will post a video on Thursday with his answer.

[Explore past challenges here and here.]