It is the rarity for music groups to keep the same lineup over the course of it's history. Reasons include (but not exclusive to):
Sick of being poor
Sick of traveling
Sick of bandmates
Sick of abusing substances
Sick of corporate runarounds
Sick of being sick
Sick of being unknown
Sick of being known
Sick of being underapprcated
Sick of being bad
Any or all of those have put the kibosh on the dreams or harmony of a musical group. Some groups change rarely, but just the sheer number of years on the road will bring about change. Others can change lineups so much that reunions of the bands in question come with "Hi I'm..." badges. Changes can bring about differences, while some bring no changes in the sound whatsoever.
However, every so often changes will occur that will totally take a band into another genre altogether, One latter day example was with the band Genesis. Peter Gabriel leaving the band and Phil Collins taking lead vocals move the band from art/progressive rock to pop rock in very short order. Another was the disco group Exile morphing into a popular country band. Today however, we consider The American Breed.
The band's heyday was in 1967-68 with five songs on the charts, by far the best known being, "Bend Me, Shape Me" in December of 67.
The hits stopped coming by the end of 68 which began a cycle of changes that only kept guitarist Al Ciner and keyboard player, Kevin Murphy. Not only that, but they went through several name changes...they went as The Breed, then as Smoke. Went through a couple of lead vocal changes as well until settling on an 18 year old fireball of a singer named Chaka Kahn. By this time (1973) they had also changed the name to Ask Rufus. By 1974 this core wound up with a number three hit and success in a totally different genre. A long hard climb from pop to R&B, but a very successful one....