Willkommen in der Vodafone Community
Thank you for clarification. As written Changing your tariff is earliest 3 month before expiring day possible.
In 09.2018 you accepted the tariff. You knew that the duration is at least 24 month. It's not possible to change the tariff now. You have to wait at least til 06.2020. I know that's not the answer you want to read.
I guess also 06.2020 is also the date I can state my desire to leave? Which obviously will be the case, since I am not satisfied with what I get. Yes, you are right, I accepted the prolongation, but because of mainly the fact I missed my leaving date with 1 day, which I will never understand how strict are those mobile phone contracts in Germany. I will wait my time