I think they are both wrong. You want "didn't use to": "He used to be so friendly." "He didn't use to be so grouchy."

If I understand your question, "didn't used to" is the correct phrase. You will need to use "did" instead of "do" because "used to" is past tense. Example: I didn't used to live in town. However, it is a bit of an awkward phrase. Using that example I would probably say something like "I didn't live in town before last May." instead.

Can you please rephrase that question.

Sorry, I wanted to know the negative of 'I used to play the piano', and thought it would be 'I didn't use to play the piano'. Now I know it is weird to use 'didn't use to verb' regardless of the grammar's correctness.

I don’t understand the question.