There have been a few.
Derek Johnstone being the most obvious. Matt Elliott. Chris Sutton was used as a centre half at Norwich City. Paul Hegarty, Jack Copland and John Clark all started out as centre forwards. Even Barry McLaughlin made his Saints debut as a centre forward!
John Charles, of Leeds Utd, Juventus and Wales was used as both a centre half and a centre forward. 370 goals in 715 league appearances!
Going back further in time, the old Saints centre half, Willie Telfer, was utilised as a centre forward on a number of occasions.