Do seismic waves change speed?
Yes, and that fact plays a major role in our study of them.
Any time seismic waves pass into a different material or different phases, it will liky change speed. For example, when seismic waves pass from the crust into the upper mantke, they speed up and also get refracted because of this change in speed. The discovery of this particuar discontinuity (now called the Mohorovicic discontinuity or Moho), in 1909 by the Croatian scientist Andrija Mohorovicic, is a seminal event in the development of seismology and our understanding of the Earth's structure (see http://geology.com/articles/mohorovicic-discontinuity.shtml).