A broom-maker gathers straw and and trims it and binds it together, then finds a long stick and fastens the straw to one end of the stick. What results is something that never existed before, that has no history. Being incorporated into a broom remains part of the history of the straw, and similarly for the stick. But the history of the broom begins when the broom-maker fastens the final knot. It is a new creation.
When people become acquainted, an acquaintanceship is born. Although not a material thing like a broom, it will also have its own history, distinct from the histories of the two people involved. So also marriages and partnerships and joint ventures of all kinds.
In a sense, everything we do is a new creation, with its own history. When I drive to the store or deliver a sermon or smoke a pipe, what is done reverberates through creation from that time forward in its own historical arc. But the infinity of these new creations are, so to say, sloughings from me, expanding out from me they way the water wake does from a boat, or the vapor trail from a plane. They are not separate elements brought together into something new, with its own identity.
When someone accepts Christ as Savior and Lord, a new relationship is born. That is the new creation of which Paul speaks. It is not the person undone or wiped out, and then re-created. The straw does not cease being straw nor the stick stick when, together, they form a broom. Instead, together, they form a new identity, which then begins upon its own life history.