My situation was nearly the same, rather then 20 lessons it was more like 10. I have to say it was the best thing I could have done. I didnt find my new instructor went over things i'd already learnt unless he knew I weren't good enough at them.
How close do you feel you are to taking a test? If you feel you're only 50% there then it could be worth getting a new instructor. If it's more like, 85%, then may be worth just sticking with it. There's nothing worse then not liking the person trying to teach you something, it will result in you not wanting to learn from them, or more importantly, not wanting to learn at all. I hated my instructor so much I gave it a year before I could face another .... I wish I had of just changed instantly with hindsight.
To be honest id say about 50%. So it is probably worth getting a new one. I actually dread having lessons with him and as you said, its going to result in me not wanting to learn and im obviously going to learn more if I enjoy it.
Thanks for your input mate