...keep in mind that if you're planning to do what i think you're going to do, you can't check bags and you can't purchase as a round trip since your remaining flights (DTW-STL, STL-DTW, DTW-BOS) would be cancelled.

there's also the "threat" of getting your FF account closed or frozen, though that seems more of an idle threat.

oh... and if your flight from BOS-DTW cancels, they'll route you BOS-JFK-STL or BOS-CVG-STL and you end up somewhere you don't want to be.

otherwise, carry on.