At the moment, you cannot change your mind about learning a skill while you are in the midst of it.
I've stumbled across one of the most egregious examples of this:
I'm in the middle of learning Mining V. My 'time to learn' is 21 days, 13 hours. I have 17 days, 11 hours remaining before I finished the skill.
When the new skills came out, I really wanted them far more than I wanted mining, which I only started last week because I'd finished everything else. So I paused mining to learn the distilling/tincture/potion tree.
I was then going to get all the Unlearnings and get rid of my mining tree. Unfortunately, I cannot do that. I first have to spend 17.5 more days learning Mining V before I can start unlearning it. If this isn't a bug, then it's a feature that needs to be re-thought.