Of course, for your case, if you can not get what u want, the feeling to keep working in the orignal department will not be so good. So think about it over: whether u 心甘情愿 lose somthing instead of fighting to achieve something ( which means, you may have it or not in the end)
the best way would be 找HR帮你，
because HR also has the rights to 人事调动 if it is inside one company
besides, if a 外人 ( like lawyer, MOM, etc...) even if they can help u to achieve what you want, you still need to stay in the same company and get a lot of "敌视你的人" , it is always no good to have such people arround you. And what's more, maybe the 外人 can not help you at all.
if u want HR to help, u need to presnet the idea that if you can be transferred to another dept, your contribution to the company is much better than you stay with the current one, the profit u can bring to the company is much much more. (focus on what u can bring to the company)
2ndly, talk to ur own dept in a nice way, and give up a bit. like u can work for them to complete the tasks in your hands first and then transfer to another dept. Or maybe u can work for them and the other dept at the same time, until ur current dept finds the suitable person. (focus on that your leaving will not bring them any incovinence but goodness to all)
One friend of mine had met the same situation before and she successfully transferred. but note, she is not working here. so I recall what she told me that time and present the thoughts with you. Not sure if it helps but I wish u good luck
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