The 33 year old will continue to play but offers boss Ricki Herbert some experience behind the scenes.
"The character, application and dedication Chris has shown since joining the Phoenix has been second to none, and I know he will bring those same qualities to his new role," Herbert said.
"His involvement on the coaching side will relate directly to the front third of the pitch, because he has a good understanding and feel for players who operate in that area.
"He will remain a member of the playing staff and be selected according to performance, the same as any player at the club.
"Chris has been respected by other members of the squad, and from discussions I have had with senior players in coming to this decision it is evident he will have their support in his new role."
Greenacre said Herbert's offer was too good to refuse.
"It is a fantastic opportunity and I would have been crazy not to jump at it," Greenacre said.
"All my close friends know this is what I want to do when I finish playing.
"Hopefully I can still play a part as a player because I'm still in good nick physically. I enjoy training and going into work every day just as much as I did as a 17-year-old at Man City.
"I'll continue to work day in day out and will be treated the same as everyone else while doing so. There will be no special privileges but when Ricki needs a hand I am capable of stepping up to the plate.”