What exactly is it that you could do with the 17-55 that you can't with the 24-105? The 24-105 on a full frame body is wider at the wide end and longer at the long end than the 17-55 on a 1.6 fovcf body. It is also faster: f/4 on a full frame body is like f/2.5 on a 1.6 fovcf body, both in terms of dof and light gathering.