I believe that once was true, though so much has changed since then that it is almost certainly no longer true.
Way back at Comic-Con 2007 they announced they intended to release
a Narnia film every May starting with Prince Caspian in May 2008 (
VDT was scheduled for a May 1st 2009 release). Such a tight schedule would of required a different director for each movie. In April 2008 they even announced they had a
short-list of potential directors for Silver Chair, but of course that was all before everything started going wrong for the Narnia series. Nothing is certain anymore.