Final Cut Pro X has two functions that are integral to editing, expecially asynchronously, in which it allows you to retain all your edits in your project while replacing footage, without causing major issue. This tool can be used in a host of other ways as well, but this is just one great use of it.
You can think of this as wrapping your footage up in something that behaves exactly as a clip would, and then editing using that wrapping.
To set yourself up, what you'll want to do is to right click the footage you'd like to use and select New Compound Clip...
This will create a new clip that is identical to the old one, with the one addition that you can now double click and open that compound clip to reveal a new timeline.
The power of using this technique is in cases where you might be working with low quality footage that you know you'll be able to replace with higher quality originals. Once you've replaced the footage, all your edits in the Project are going to remain the same, just applied to the higher quality footage.
Another great use-case for this is when you're planning on applying a global effect to the composition(like tiling, animations, or text overlays). You can take a blank color box and put it in the compound clip, to be replaced with footage later on!