[Benjamin Petric]

I think the best way to move a Chapter marker in FCP is to position your playhead in your timeline exactly where your Chapter marker used to be copy its timecode then locate the chapter marker that you wanna place in that specific location,make sure that your marker is Highlighted and then hold down OPTION++M key and the marker’s window will pop up .In The Start box paste your new timecode and click OK.

Hope this will help!