There are actually two worlds here: one old and one new.
Craig Bartholomew and Michael Goheen write,
Two great figures stand at the entrances to two worlds: Adam stands at the gate of the old world, Jesus at the gate of the new. Adam’s first sin inaugurated the old age and brought sin, death, and condemnation. Now in Jesus a new day of righteousness, life, and justification has come (Romans 5:12–21). If we are “in Adam,” we are part of the old age and under its sway. But if we are “in Christ,” we are part of the age to come and can already experience God’s life-giving power. (The Drama of Scripture, 189).
This is the context in which we read Paul’s amazing words in 2 Corinthians 5:17. If you are in Christ you are a new creation. And that means you have been transferred from the old creation that has passed away. Being in Christ means a switch of kingdom residence and evidences something that transcends our individual lives (Colossians 1:13–14).
God’s new world has broken in upon the old. Jesus is making all things new. And we are part of his work.