It depends on the supervisor -
But in scientific PhDs its very unlikely that you will finish doing the exact project you started and theoretically after 1st year its up to you what direction it should go - although it will be within the confines of your supervisors experimental area usually
So the point is, it is possible to change
However if it is an industry funded one - then they tend to be very directed and usually have to do what they were funded for
hope this helps
Good luck
S