Online definition of "apostate": "A person who renounces a religious or political belief or principle."
JW Simplified WT defiinition: "People who rebel against true worship and abandon it." (page 11)
The dictionary definition would mean that anyone leaving another religion to become a JW is an apostate. The JW definition boils it down to anyone who leaves Jehovah's Witnesses because they disagree with that religion's teachings.