Ferg — born Stacy Ann Ferguson — has filed legal papers requesting to officially change her name to Fergie Duhamel … roughly 4-and-a-half years after marrying Josh Duhamel.

The reason for wanting to change her last name is obvious … but in the docs, filed in L.A. County Superior Court, Ferg’s lawyer says she wants to swap Stacy for Fergie in order to “conform to the first name which she is commonly and professionally known as, and has been for many years.”

In other words … no one’s called her “Stacy” in YEARS.

In order for the court to grant her request, Ferg … or Stacy (for now) … must publish a notice in a local newspaper for four consecutive weeks (just like Ron Artest was required to do in order to become Metta World Peace).

She’ll most likely get her wish if she completes her task by August 16 — when a hearing is scheduled for the matter.