I don't know what changes there are. I mostly faced two types of changes.
- developing class; when a class is learning Aikido, sensei usually starts to show the basic moves. Later he will put accent on right body holding and move and somewhen it will get dynamic. Although it is always been the same technique, it looks quite different and I always have the impression to start again from early beginning.
- variations; there are thousands of variations of the basic techniques in Aikido and each one has to find out, which variation matches best his body and strength in which situation. So it is quite typical, that each time sensei comes from a seminar, s/he brings some fresh variations. And very often they seem to be very different from everything you've learned before.. Just after getting used to the different form and rhythm, you will see, that it is in fact only a slight variation of the "old" form.
So yes, in any case it is hard, but try your best. That is the way you learn flexibility.