I was on 20mg of Olanzapine for a few years. I dropped my dose down to ten thinking that I was going to be fine, as I had done that in the past. However a few months down the line I started getting symptoms again, and now I am on 10mg of Olanzapine, and 30mg of Abilify.
You may be able to drop your dose and not experience any symptoms, however I'd make sure you lower the dose with a doctor. 20mg is the highest recommended dose, so I can imagine you must have had it rough in the past.
Good luck, I hope you find a solution. For me it's another medication, but I hope you find the best way for you as quick as possible.
I must say that Olazapine is in my opinion, a nasty drug, which is why I am glad to be moving onto something different.