Jack isn't changing his job occupation. Just his title. Don't worry he'll still kill giant. It won't be as severe! *Shrugs*
For some, undisclosed, reason Warner Brothers is tweaking the title to Bryan Singer's fantasy-actioner starring Nicholas Hoult from Jack the Giant Killer to Jack the Giant Slayer. That will all make the difference in the world and make everyone forget the terrible trailer, its sudden change from June 2012 to March 2013 and those "Jack the Company Killer" quibbles. Speaking of, the studio also is moving up the release by three weeks to March 1 from the 22nd.
Prove me wrong, Bryan, but this looks like a stinker!