Did the Sabbath moving from the seventh day to the first day happen after the resurrection or at some other time?
Neither. The Sabbath did not move. There is no Christian “Sabbath.” Sunday is definitely not a Sabbath. Nowhere in the Bible is there even a hint that Sunday is a Sabbath. In fact, one could argue that we are not even commanded to meet on “the first day of the week.” We meet on “the eighth day” more because of example than because of command. There is a kind of “Sabbath” in Christianity. Our Sabbath is heaven. Until then, we have no rest. This is the message of Hebrews 4:1-11. Technically, the Sabbath was not ended or replaced. It was “fulfilled” by Jesus, which means that we do not have to observe it (Matthew 5:17). If a Christian wants to observe the Jewish Sabbath they may. I believe that Paul did so. He authorized us to do so if we like (Colossians 2:16), but also commands that we not force it on anyone. If you want to observe a personal Sabbath, perhaps on Sunday, you are free to do so, but remember that this is not a Christian teaching and should not be taught as if it is.
I hope this helps!