"By what authority does anyone have to change the Christ ordained role of believers from witnesses of Jesus to Jehovah Witnesses?"
The term 'witnesses' or 'witness' is different in the contexts.
When Jesus said in Acts 1:8 "you will be witnesses of me in Jerusalem, all Ju·deʹa and Sa·marʹi·a and to the most distant part of the earth" He was referring to the teaching of him and the spreading of the news, because it was new. Now Jesus isn't new so there is no good news or gospel.
However the use of the term Jehovah's Witnesses was from Isaiah 43:10 " “You are my witnesses,” declares Jehovah,“Yes, my servant whom I have chosen,"
(though it is odd that the reference is to Acts 1:8)
vs 12 "“I am the One who declared and saved and made knownWhen there was no foreign god among you. So you are my witnesses,” declares Jehovah, “and I am God."
That has to do with who God is and his worshippers and how they live. Not preaching or spreading news.