It is possible to replace the cables without replacing the housings. But why bother? The housings trap dirt and water causing corrosion and excess friction that effects shifting performance. Replacing Shimano housings is an easy job. If the old housings are of the correct length simply cut the new ones the same length. If you buy the Shimano kit the short housing found at the rear derailleur is pre-cut to the correct length. This is the most critical housing for shifting performance because it is subject to more dirt and crud off of the road and because the curvature of the housing needs to be just right for best performance.
Replacing the housings is more important than the cables so far as shifting performance is concerned.

Al