Henka waza is what is commonly reffered to as a changed technique, in such a manner that i.e. starts with one technique but halfway one changes to another.
Jiyu oyogi is what is sometimes reffered to as personal technique or the application of that moment, i.e. doing what happens. This implies often that a student has extensively studied Kihon waza Kihon Gi and from the insight gained there comes to his own creative form of aikido. In that respect is jiyu oyogi the "form" that enbodies the idea of takemusu aiki.