Hey @Marie
On your screenshot above, you are trying to move subtasks as subtasks of another tasks but if I have several tasks (not subtasks) selected at once, I can’t find the option (…/Advanced/Make a subtask of) - screenshot attached.
Or am I mistaken?
Best,
Gilles
Got you @Gilles! You’re correct, it’s not possible at the moment to make several tasks subtasks of another task; sounds like a good opportunity for a new thread in the #productfeedback category 
It got my vote.
Dear @Marie,
I tried your suggested method to move some subtasks from one task to another but I have faced with an issue.
If I select the subtasks and after that press the Make a Subtask of button on the parent task (Test in your example)
than the parent task (Test) will be the subtask and the subtasks will be sub-subtasks.
My question is that is it possible to move only the subtasks?
Thank you in advance!
Regards,
Adrian
Hey @adrian.szlama, I’m stepping in here for Marie!
I am sorry to hear that you are running into issues with this, I completely understand what you mean.
In order to move Subtasks under a different Parent-Task, you would currently have to click into each Subtask to access the Subtasks menu, you cannot currently move multiple Subtasks under a different Task through the dropdown menu.
An easier way to do this may be to simply drag the Subtasks into the Project itself and then under the new Parent-Task. You can learn more about this here. Let me know if you have any follow-up question!
WAT ? Are you kidding me? 
it is terrible, sometimes i need to move hundred subtasks
An easier way to do this may be to simply drag the Subtasks into the Project itself and then under the new Parent-Task. You can learn more about this here . Let me know if you have any follow-up question!
It might be easier if asana will support drag and drop to another window
Am glad this feature has been added. I have had a great deal of challenge to move tasks around from being a main task to a subtask in board view. This solves my problem. Thanks for this feature. I have been late in seeing this feature but today it has helped me a lot.
Agreed. Very clumsy and inefficient work-around use of this feature. Right now it is simply not possible to do this in bulk, only one by one.
An easier way to do this may be to simply drag the Subtasks into the Project itself and then under the new Parent-Task. You can learn more about this here . Let me know if you have any follow-up question!
How do we do this in bulk though? Whilst you can select & move multiple subtasks into the Project itself, it is not possible to then select & move these “subtasks” (which have now been turned into “tasks” under the project) under the new Parent-Task.
Hey @Brandon_Truong-Phan, my apologies for the late reply. It would currently not be possible to multi-select Tasks and make them Subtasks of another Tasks though I can certainly understand how this could be really helpful. If you have not done so already I would recommend creating a thread with your feedback in our Product Feedback category!
Is it possible to convert a subtask into a proper task?
You can open the parent task, and drag and drop the subtask in the project and will become a task.
If you drop it in between two other tasks (not on the section name) it will brake free from the parent task and will become a task in the project.
If you want it to be a task in the project but keep it as a subtask of the parent task:
