Droopy in front, Dad in back. When I have a pair of favorite blue jeans that are nearing the end of their useful life, I now extend them a few years with transplants from "younger" jeans. By this time they are work-around-the-house or trip-to-hardware-store appropriate. I used to patch the knee holes but by this time the front thigh's fabric is usually so thin the patch does not have much to hold on to and a hole just appears higher up the leg. A few years back I tried a more radical approach of transplanting the entire front of each leg of the pant from the waist to below the knee. It was a success so now I do it regularly. This process provides 8 immediate benefits:

a new lap and knees to start wearing out
retains the great fit of favorite jeans
a second set of front pockets lower down the legs
the illusion of droopy pant in front while retaining the traditional "dad" seat and fit
the lower pockets are a comfortable place for bulky stuff like cell phones that can be uncomfortable in a top front jean pocket
the opportunity to indignantly say "my eyes are up here" to anyone that comments on the pant while I am wearing them
the ability to scratch further down your leg while keeping your hands in your pocket
a conversation starter with people that find the pant funny and/or interesting

Step 1: Materials and Tools

Disclaimers:

I am a self taught sewer that sews mainly to repair things or create things where a perfect finished appearance is not the top priority.
I choose to go with "pant" vs. "pants" because when I looked it up it has an earlier heritage. Also, when I tried the possessive for pants it got awkward.

In this instructable the parts I am preserving/repairing are the "host" and the pair I am parting out are the "donor".

Materials and tools:

a pair of jeans you want to extend the life of (host)
a pair of jeans you have no use for (donor), preferably newer than the host pair. Size doesn't matter. For me these are jeans that don't or never fit right, or are un-returnable "bargains". (like you get home to find they have a button fly)
sewing machine
suitably colored thread
scissors
seam ripper (or a pair of scissor and patience)
straight pins

Step 2: Prepare the Host Pair

First, gain access to the front of the legs. This is best done by removing the stitches of the outseam from the rivet of the front pocket down to the hem of the pant leg. Use a seam ripper for this if you have one. The pictures show before, one leg opened, and both legs opened.

Step 3: Remove the Transplants From the Donor

Now separate the leg fronts from the donor pair. I prefer to remove the stitches completely from the inseam and outseam of both legs. I have also done this by using scissors and just cutting close to the inseam and outseam but removing the stitches gives you more fabric to work with and hence more room for adjustment. Leave the zipper in or take out, doesn't matter, your call. The pictures show seams removed, zipper removed, waist button retained. Removing the stitches helps when attaching these to the host pant because the inseam can be sewn as is, since the fabric is already folded under and ready for stitching.

Step 4: Trim and Tack the Donor Legs to the Host

Orientation of the transplanted leg. The waist of the donated leg should be placed at about 1/2 the way between the bottom of the zipper and the bottom of the crotch. I find this makes donor pockets easily reachable and generously covers the part of the host leg that needs it the most. The finished outseam of the donor leg should be about 1 cm or 1/2" from the outseam of the host pant. If you removed the stitches vs. cutting out the donor leg this seam is ready for pinning to the host. If not, fold it under for about 1/2" or 1 cm so that cut fabric is not what shows.

The inseam side of the donor leg should be cut where it lies above the host inseam. That way when it is folded under it will have about the same distance from the host inseam as the outseam side.

I shorten the donor leg by cutting off enough material to expose about 2-4" or 5 cm. There is no reason for this distance. For my next pair, I may cut them to the same length as the host pant. Fold under the cut edge as on the inseam. Use straight pins to secure the seams of the donor leg as you want it sewn.

Here you can decide if you want the feel of the donor pocket having access to your leg. If so make a cut in the host pant thigh so the pocket of the donor pant will fit through the cut in the host pant. Then you can scratch your leg halfway down your thigh with your hand in the donor pocket.

Step 5: Sew

Thread the sewing machine with the thread you chose. If the donor pant are dark, like mine, then navy blue to black will suffice. I am not sure about how much to explain here so I'll keep it simple. With the donor side facing up sew all around the donor leg attaching it to the host pant. If there are any holes in the host leg sew the edges of the hole to the donor leg as well. If not your toes will catch on the hole when putting them on. Spots where four or more layers of fabric occur I just skip. Its usually only about 1/2" anyway. I recall trying to sew some of those and often breaking needles. Ditto for getting too close to pocket rivets.

Close up the patient. Close up the host leg (with the donor leg sewn on) so that the host leg is inside out. We are now sewing back up the outseam we first unstitched in the first step of this operation. Match up the two sides of the seam as they were before they were unstitched and pin them together. Sew the seam closed. Remove pins.

(Memory lane: I realized how to open a leg to patch it when I recalled that girls in the early 1970's would adjust the fit of their jeans tighter by sewing inside this seam. It wasn't noticed until they needed to make them less tight and they removed the seam they added. Then there would be a strip of darker fabric down each outseam where the fabric that was hidden had not faded. This was before pre-washed jeans that come faded and don't shrink with washings.)

Repeat the process with the other leg. This is also a good time to repair/re-enforce the seat of the host pant. In my experience where the tops of the rear pockets attach to the pant is where the fabric fails next.