The most common change is length of bodice, either shorter or longer. People vary in height, people vary in their length of waist. Most people already know if they are short or long waisted. But in case you don't, put on your stays, tie a piece of elastic around your waist and move around a bit. The elastic will land at your waist. The bodice pattern should land about 1/2 inch below your waist, (the 1/2 inch is your seam allowance). If you need more length follow these simple steps.
Trace your bodice back and front patterns onto a piece of tracing paper or interfacing, transfer the line indicated to lengthen or shorten. I use oak tag, but any paper will do. Cut your pattern pieces along the lengthen/shorten line on the traced pattern. Tracing the pattern is very important, you want to make sure the original is untouched for future reference.
Spread the pieces no more than 1/4 apart. I like to use graph paper as the 1/4 inch marks makes this so easy. If you need to lengthen even more than 1/4 inch, slash your pattern apart again, draw another line 2 inches above the first , cut and spread it apart another 1/4 inch. This helps preserve the curve of the pattern pieces.
Trace around the spread out pattern onto the paper underneath and you now have your new longer bodice pattern. Mark the pattern with the date, and what you did! This will help you a year from now remember your changes.
What ever you do to one of the bodice pieces, do the same to the other. So altering the front means you alter the back or the side seams will not line up.
To shorten, do the exact same thing, slash along the line and instead of moving your pattern apart, slide the top of the pattern over the bottom 1/4 inch. Again, if you need to shorten more, make another line two inches above the first, cut and move in the same manner. Make sure you add or subtract whatever you did here, to your center back seam as noted in the pattern.
This is the same technique to use when lengthening or shortening the sleeve or the shoulder strap.