User blog comment:KidVegeta/Canonicity/@comment-3097771-20101130082822/@comment-3097771-20101130093059

Oh, I think he planned on transforming, just not to start. Remember this form was new to him, and he was inexperienced with it. You should also know that the Yadrat training, if there even was any, was minimal because Goku was only barely stronger than he was (base) against Frieza, when he arrived back on Earth. So to say that he mastered the form, therein is false. I think he was just caught off guard by Cooler and couldn't transform in time (remember it takes far longer to transform in the heat of battle, then it does when not under pressure). It also would be harder to transform, if he had not had much practice (much like how long it took him to transform to SS3 when he had little practice with it). -KidVegeta