Since you are PM and both tickets were booked using your miles, why don't you just make the change now (assuming award seats are available? You are allowed a free stopover on an award ticket. If your stay in BOS is <24 hours then it is considered a layover; if your stay in BOS is 24 hours or greater, it is considered a stopover.

If you can't get award seats in advance, totally forget SDC, since as was stated above, SDC will not work on any portion of an international itinerary, even if the portion you want to SDC from/to is wholly within the US.