If you don't want to lose sizing on the commercial pattern piece, this is what I do based on the pattern drafting method I was taught to do in fashion school:
1. Cut out the pieces just to separate them. Don't cut on the lines.
2. Place on your fabric with pins
3. Using a long ruler (the ones we use in school look like this
and cost about $5.99, but you can use any flat ruler with no bevels or traditional angled edges) and chalk, put the edge of the ruler on the line of the pattern piece, fold up the pattern paper over the ruler, and swipe the chalk along the ruler. By shifting and angling the ruler, you can get curves even though a ruler is straight.
It seems counter-intuitive because a ruler is usually used for straight lines and measuring straight lines, but this is the method I was taught in pattern drafting, when we'd be given a sloper or block with no seam allowances, and had to measure and draw our own seam allowance after altering the block. When doing a curve, you just pivot the ruler in tiny increments, and draw a bunch of lines. Where they meet, you get a smooth curve.