Well, after reviewing the costs involved in having contractor do the work, I decided to evaluate doing it myself. I only have on problem: Plaster.
The finished ceilings on the first floor are all plaster. As a result, a lot of plaster had oozed between the lath strips between the joists upstairs. Can I raise the sill (with 1" spacers) or do I have to notch out a trough in the oozed plaster for the sister joist to ride in?