Can we ask Allah to change our taqdeer/ Qadr.
Can we ask Him to change the heart of an oppressor?
Can we ask Allah to do such and such at a specific time?
I believe the answer to your question in short is yes. But there is no need really to ask God to change our destinations. Because, that is something which we can do ourselves. Like for example, take a look at the moment when that person is in grave and he died as a disbeliever and then the Angels says to him that if you were to die as a believer, then today you would be saved. Which indicates that you can change your destination by yourself but most importantly, you also need to supplicate so that God can help you achieve what you want in future and what you're planning.
Regarding the second question, I don't think it is permitted. Because, that is something which is not in our concern, all what we can do is to punish severely the oppressor so he can taste the evil consequence. But since you're talkin' about asking God for this, I don't see any problem in it. That is I believe to be praiseworthy supplication.
The last question, that depends what you're asking. If you ask for death, then that is forbidden and anything else besides it is permissible. But you also need to work yourself to achieve the goal you proposed. For example, yes you can ask God to help you pass the exam this year, but you have to do also your part otherwise, your dua won't be answered, because God does not help lazy people. He helps those who work hard and strive for that thing which they proposed to get it in future, but that is also depends whether that goal is haram or halal.
I hope I answered correctly. If anyone noticed a mistake in my answer, let me know the mistake.
