Here are the piece diagrams from Past Patterns 803. These are pretty different from those of M4548. The most obvious difference is in the front and back linings. Note that the back lining is cut deeply into the pattern piece from the side, and the front lining wraps pretty far around the side of the torso (you can tell this by looking at the armscye, which wraps around the underarm). Also, note the elongated shoulderline.
Here are the pattern pieces from M4548. I'm actually using pieces cut from a scan of the pattern illustrations to do this in small-scale because it was easier to photograph. 1A is the bodice front, which has an attached neck facing. 2A is the center back, cut on the fold, and 3A is the side-back.
Here, I cut off the attached facing and removed the "seam allowances". (Ignore the drawn-in darts. I was brainstorming.) I really only needed to cut away the seam allowances from the complementary curved edges of 2A and 3A.
I changed the shape of 2A and 3A using instructions I found online for altering darts into princess seams. I basically turned a princess seam into darts, moved the darts, and then altered them back into princess seams.
2A and 3A joined along the lower curve.
The curve altered to turn it into a plain dart.
New dart marked in the approximate location where I will want 3A's new curve to be.
Old dart folded out, new dart opened up.
New dart extended into a new curve.
3A cut down into a new piece.
3A grafted onto 1A to create the extended wrap-around side, more similar to that seen on the Round Dress pattern.
Shoulderline rotated backwards by shaving a piece off of the bodice back and attaching it to the bodice front.
This is a bit of a cop-out, but the pieces actually ended up looking more like the Round Dress pieces once I did them in full-scale. More on that later.
I also compared it to Simplicity 4727. I figured that bloody pattern owed me after all the time I spent on it. The shapes are quite different but the finished sizes would be about the same, which is a good sign.