You do change horses in midstream...
...when the horse you're on already has you up to your eyeballs in cold water and is determined to head for an even deeper part of the stream. The Spanish know this; will Americans? Unfortunately, the closest horse to jump to is also looking longingly towards the deep water. Do you risk drowning completely while looking for a truly suitable mount, or just jump on the nearest one and hope for the best? That is THE question of 2004.

Josh Marshall has a more colorful analogy.