Create a new 24fps QT master file from the original 25fps master.

- When you use the "25@24" method, which will play all your 25 frames on the 24fps timeline and make your program longer, FCP X will be perfect for this. Just create a new 24fps project, drop the 25fps QT master onto the 24fps timeline and choose "Automatic Speed". The overall program will be 4% longer as your video and audio will be slowed down by 96%.

When you do this in FCP X, this will NOT alter the pitch of your audio as long as you have "Preserve Pitch" checked in the FCP X Retiming options pane: