Religions do not change. People may change the way they perceive or understand their own devotion to a religion but the principles and laws of any particular religion cannot change.
Why does that conflict with what you understand? It seems quite clear what I wrote: religions do not change but the way people view/distort/ change/alter/interpret religions does. Then they start new "offshoots" as you put it. But let me repeat the main point here and that is:
Islam is not a religion!