If you change the framerate of the scene, the keys should get shuffled around properly without you needing to do any tedious deleting and shifting. So the solution might be as simple as selecting '24' for your framerate (Preferences -> Settings).
If that's not going to work for some reason, you can scale your animation curves in the Graph Editor. Choose Edit -> Scale  (options box) to specify a 0.8 horizontal scale with a pivot of 0.
I think one of those should work... post again if they don't.
you can also slide keyframes in the graph editor by selecting them and entering either "+" or "-", then "=", then the number of frames you want them moved in the first box for the keyframe stats in the graph editor. example: +=10