If the jeans you have would fit and look better with a straight leg or tapered leg silhouette, just a few new seams and re-hemming will convert them. Once favorite clothing is renewed and repurposed, it is undoubtedly in line with a philosophy of sustainable sewing. At its most basic level, sustainability means meeting the needs of the present without compromising the ability of future generations to meet their own needs; the process of clothing alterations has always met that goal.
Denim jeans usually have top stitched side seams causing this type of alteration to require a few additional steps to reassemble the side seams. Slacks made of stretchy fabrics can easily be altered as well if care is taken to not excessively stretch the side seams when altering. Dress slacks, chinos or khakis work best when using this alteration technique.
Since the slacks to be altered are presumed to fit well at the waist, hips and thighs, just the part of the pants from the knee down, on both outer and inner side seams will be affected. Note: just taking in the outside or just the inside seam of each pant leg causes the slacks to lose the fabricís straight of grain line from which they were originally cut, this can cause the pants legs to eventually twist awkwardly.
Try the slacks on and place a pin at the inside and outside side seams of each knee area. Turn inside out and transfer marking pins to the inside. Open up the pant leg hems and press flat. Place straight pins from the knee area down each side of the pant leg to the pant leg bottom to mark where the new straighter seam lines will be. Baste along the pinned area, remove pins, turn slacks to right side out and try on to see if the new straight or slightly tapered leg is the look desired. Turn inside out, stitch, tapering into the existing side seams at the knee area, trim the seam to remove the old seam, and press seam open. For previously top-stitched denims, recreate the original top-stitching on the side seams. Re-hem to finish.
Some things to consider:
As you pin the new seam, consider that taking in just 1/2" from each pant leg side seam from the knee area tapering inward toward the hem area to produce the straighter leg will reduce that area width greatly. The new seam on each side should be taken in slightly less than anticipated for careful results. The seam line can always be adjusted slightly more if necessary.
If altering the pants legs to produce the tighter skinny jeans look, make sure there is enough room around the ankle areas to allow for the insertion of the feet.
The new edges of the seam allowance may be stitched to prevent any fraying. Zigzag each new edge or use an overcast stitch or serge the edges together if the original seam edges were serged too. Using pinking shears on the new seam edges may work to prevent fraying rather than stitching as well.
By re-stitching a new seam line from just the knee area down to the opened hem on both the outside and inside of each pant leg, the pant legs will be taken in evenly keeping the straight grain of the fabric as before that allows the pant legs to hang straight.
Sew happy, sew inspired.