I understood henka wasa to cover any change in the technique.
You change due to tactical consideration.
Ie you started ikkio, someone was coming so you straighten his arm and rokkyo him.
You change du to uke resistance
He straighten his arm to resist ikkio, hence rokkyo.
I think in includes the case where I expected the arm straitening or because I have awase and I felt the defence and adapted to it.
I have the same understanding of keishi wasa as Chris
understand henka waza to cover any change in the technique that leads to the same technique