|This is how RTW jeans are shortened in NYC, so you get to keep that 'worn' hem...you will need to buy jeans thread.
 Measure finished length you want jeans to be. Cut along that line. Make sure you do not cut through the existing old hem.
 Take the part you cut off [the old hem], and cut about 1/2" seam allowance above the old hem.
 Remove old hem stitching.
 Open the inseam and outseam on the old hem. Measure to match width to newly cut bottom of jeans. Re-stitch the hem's inseam and outseam with a straight stitch [no serging, too bulky), and press these seams open.
 Press under the 1/2" allowance you cut above the old hem, to form the old hem into a binding.
 Stitch these "bindings/hems" to encase the raw edges on the bottom of the jeans with jeans thread. Make sure you match the side seams.
[Nordstrom's charges $19 to do this!]
Edited to add image link above.