I am surprised that no one has mentioned Paul Stewart and his transformation from average centre forward to international midfielder!!
When Bill Nicholson bought Mike England off Blackburn Rovers, he was a centre-forward. But Bill saw the potential in turning him into a central defender and it was in this role that he became arguably the best centre-half in Britain. Oh... and Steve Perryman started out as an attacking midfielder (inside forward, as they were called back then) and gradually dropped deeper as his career progessed, ending-up as a central defender.
Demba Ba got rejected from various club trials (including a lower league Swansea) as a Centre back, now he's one of the better strikers in the league. Then there's my namesake. And Keane changed from a lifelong Wolves fan to a lifelong Coventry fan to a lifelong Inter fan to a lifelong Leeds fan to a lifelong Spurs fan to a lifelong Liverpool fan to a lifelong Spurs fan to a lifelong Celtic fan to a lifelong West Ham fan to a lifelong Galaxy fan to a lifelong Villa fan.
Might be an idea to specify which one of the two. One we all agree with, if it's the other you're on your own
Didn't our own Mr Hoddle play sweeper toward the end of his career? With Swindon? Dion Dublin played centre half and forward.