One of our regular clients came to us with an existing animation, created by an American agency. They wanted to change an element of the video, to make it more specific to the product they were selling in the UK. Luckily, the animation had a very graphical, clean look, so we could unpick the elements, create a completely new sequence and tie it all together.
The tough bit turned out not to be the animation, but the audio. We had to keep the new sequence in time with the music, in terms of pace and so the audio still matched when we made the insert. Still, job done and client was very happy, no one noticed the join!