I have heard this excuse given to another customer from an lbs for not cutting down the housing, "Oh we normally dont cut the cables down cos some customers might want to use their cables for another bike" I just gave them the wtf look"
Some lbs's dont cut the cable housings cos it would involve them un-doing what the manufacturer did. One word or phrase rather...---> CBF.
1. Get a pair of pliers long nose preferably, grip the cable not too tightly, hit as close to where the pliers contact the cable outwards. This done to remove the cable end cap. Do the same for the other derailuer
2. Get an appropriate allen key(usually 5mm) undo both the bolts. Straighten the cables as much as you can.
3. Pull the aft end of the cable housing out of the mounts near the downtube, do this for the other one as well, both FD and RD cable housing.
4. Hold the housing and with the other hand push the cable out, it helps if you have a rubber band to hold the brake lever as close to the handlebar as possible exposing where the cable stop comes out, pull cable out. Repeat this process for the other cable.
5. Use this opportunity to check your cables. Depending on how you ride the order of cable degradation is usually as follows. RD cable, FD cable, front brake cable and rear brake cable. With the rear brake cable lasting the longest. Usually derailuer cables break at about 2cm from the cable end nowhere else.
6. By now your cable housing should come off with a gentle nudge. Remove cable housing end cap(usually black in colour) and cut as required doesnt matter whichever end you cut, making sure the cut length does not interrupt with handlebar rotation. There is a small white o-ring on the inside of the cable end cap, dont lose it. Of course put it back when you are done cutting.
7. Reverse the process and if you think that the RD cable housing is too long cut as well. Regarding the RD cable housing cut at the end closer to the front of the bike, only after removing the cable end cap though. You can get away with using something not as sharp on the derailuer cable housings but dont try using a blunt cable cutter on the brake cable housings or the cable itself. When making the cut, make it as swift as possible. Like what Bleve said the best tool for this job is the shimano one its blue in colour. The park tool one is also blue but a darker shade.
8. Depending on what sorta cable housing you've got some dont come prelubed so it might be wise to apply a thin film of grease over the cable before threading it back through.
9. Reverse the process and thread the cables back into the housing, both front and back. Make sure you thread the cable through the cable guide correctly, when viewed from bottom the left slot is for your RD cable, the right slot is for your FD cable. If your cables are frayed you would have a hard time threading it back through but it can be done with patience, having said this its best to use a fresh cable.
10. Secure the cables back to the FD and RD making sure to pull all possible slack with a pair of pliers in one hand and with the other hand simultaneously tighten the bolts to about 6-8nm.
11. If your limit screws and barrel adjusters are set up correctly you shouldnt have to fiddle with the limit screws. Finish up by putting on a new set of cable end caps, crimp crimp.. and you are done...
If the shifting is off then bring it in to any lbs and they should be able to do it on the spot. If you are up for it then thats another story for another time.
Hope it helps.