we are generally allowed to employ Henka waza if we botch it up or our timing is wrong (better than just stopping eh?)if uke is being an ass on purpose then we can use henka or sometimes just batter them (if their ukemi is upto it) though sensei will generally point out to them the error of their ways and why uke reacts in a certain way.
I remember training with a guy who claimed to be a boxer on his first session, the technique was ikkyo gone wrong so switch it to sankyo (so henka done on purpose) and the guy was fighting ittooth and nail, telling me that he would slow down if he was doing it etc etc. He was so busy fighting it that the sankyo really hurt him, he got the point after that (especially when sensei explained to him the importance of blending with the technique). Obviously I didnt intend to hurt him,I was just trying to make my technique work. To his credit he stuck around for a year and eventually made it to 5th kyu, though he was still a dickhead