How to change the transition point between two clips? (I.e. how to perform a rolling edit)
There doesn't seem to be an easy way to do this either in iMovie nor in Final Cut Pro (except for Multicam clips).

Even these two methods will sometimes fail when they should have succeeded because the two clips might have gotten so out of sync that iMovie no longer recognizes them as two versions of the same clip.

Method 1 - rejoin clips

One way you can do this is to:


Select both clips
Join the clips
Re-split them somewhere else
Re-apply all the filters you had


Notably, this seems to be the only way to do it in Final Cut Pro as the following method only seems to work in iMovie...



Method 2 - dragging clips

Another way to do this is to drag the clip as you did normally. Notice how there is now a "break" between the two clips:



Drag the other clip until this "break" disappears:



This functionality does not seem to be available in Final Cut Pro, so this appears to be one instance where iMovie actually helps you more than Final Cut Pro.