It must be remembered that there is nothing more difficult
to plan, more doubtful of success, nor more dangerous to
manage, than the creation of a new system.  For the
initiator has the emnity of all who would profit by the
preservation of the old institutions and merely lukewarm
defenders in those who would gain by the new ones.
                
-- Niccolo Machiavelli, 1513

See more quotes here.