Switch cast
My friend Adrian asked for a Final Cut Pro tip today. He wanted to exchange one clip for another in the timeline, while maintaining the filters he applied to it.
1. Select the new clip in the bin and Copy it (Command-C).
2. Select the clip in the timeline and paste attributes (Option-V)
3. In the dialog box, check the Content checkbox
alex4D old blog 
That was cool. Just to add on, you need to click on Content checkbox on the Audio, too, if you want to overwrite the audio.
thanx
Just following on from the tip about replacing the content of the clip – This is really useful, however when replacing the clip with another, there is no option as to what point the new clip should start from. IE I dont want it to start from the first frame of the new clip but instead start from say half way through from example – Is ther anyway therefore to replace the content but specify the start point of the new clip? Really got me stumped this one?