When they change routines/music(they routine changes are usually just to beam and floor), all the compulsory routines are changed at the same time---they don't change L4 one year and L6 the next. It takes most of the summer to have regional workshops for coaches to attend and the judges all have to get trained on the new routines. I was told they used to do the changes on the Olympic cycle(every 4 yrs), but this was too much to deal with and they now go to every 8 yrs.
I don't think they are changing anything. My DD will be working on the routines again soon for September, she is a repeat L4. That song really gets into your brain after a couple meets but the girls seem to love it & the skills that go along w/it. Wish they had music to the beam routine, my DD seems to remember routines/skills better w/music!