The former, I’m traveling tomorrow, means that you are certainly going to be traveling the following day; on the contrary, the second sentence, I am going to travel tomorrow, sounds as if that’s a pretty certain plan of yours, you are planning on – traveling the “following day and you are highly” likely to do it, but it doesn’t sound like : it is all organized already! After all, though, I would consider them interchangeable as the difference is almost indistinguishable ><