The one person who comes to mind is the Roman emperor Julian (360-63), who, though raised Christian, turned back to the Greek religion of the Homeric gods and tried to reestablish that faith as the principal religion in the Roman empire. For that reason the Christian historiography referred to him as "apostata". Since he fell in battle three years after his ascension to the throne nothing came of his efforts (and, as Christianity was already quite wide spread at the time, it would arguably be doubtful if that would have been reversible at all). A good overview about Julian and his political and religious aims is given in: Adrian Murdoch, The Last Pagan.

The Indian emperor Akbar is a possible example, insofar as he ultimately founded his own idiosyncratic and syncretic faith (Dīn-i Ilāhī), an unequivocal heresy in the context of his original Islam.