Well, there are three ways...
1) Don't do anything if this is to be executed in FCPX. The frame rate will be made to conform to whatever your project is in FCPX — it makes little to no difference what your settings are in Motion.
2) Create an entirely new project with a 24fps frame rate. Copy everything from one project to the other.
The problem with this method is if you have behaviors and rigs, everything pasted in the new project gets renamed with "xxxx copy"... and you lose associations.
3) Open the .motn document in a text editor. You might be able to use TextEdit, but I recommend BBedit (there's a free version - be careful not to sign up for any "in-app purchases"). Any text editor that handles "plain text" and will not change the filetype upon save will do.
Once you open the document in the text editor and "Find" > "ntsc". It is located in a tag section that starts <sceneSettings> and the section you need to pay attention to looks like this:
<sceneSettings>
<width>1920</width>
<height>1080</height>
<duration>300</duration>
<shouldOverrideFCDuration>0</shouldOverrideFCDuration>
<frameRate>30</frameRate>
<NTSC>0</NTSC>
Change the <NTSC> to 0 ("false" -- 29.97 will display '1' for true and this SHOULD BE 0 always — from now on! [NTSC is dead]) <frameRate> to 24 and change the <duration> to the number of frames your project will be when you change the frame rate. [This third part is not absolutely necessary -- you can correct the time difference in Motion, but it will make the change more convenient if you do it here.]
You do not have to quit Motion or even close the document when you make the change. Motion might ask you to Revert to Saved or Keep the current version -- revert to saved to load the changed version. [Save the version you have open before making any edits in a text editor.]
DO NOT - change any of the document formatting (unless you know exactly what you are doing*). All spacing (tabs and newlines) must be maintained or the document will not be read. Just change the numbers/data within the xml tags.
*it is possible to add information like a Theme, etc... you have to know what the formatting for that section is to add it.
