Sometimes things change. If your guide(s), subconscious, some entity, etc., are expressed to you as an animal, this could represent different things.
It could be a change, from what the sparrow represents to you, to what the fox represents to you.
You may have learned the lesson you were meant to learn from the sparrow, and now you have something to learn from the fox (theoretically).
The fox could be a representative of a "shadow totem" of sorts, being that it is a predator of the sparrow. It could be something about other aspects of yourself, your life, and be there to show you more about yourself.
And by no means are those the only options.
Ultimately, it comes down to looking within. Understanding what these animals mean to you, and what they mean framed within your beliefs.