Task groups are a great way to organize tasks within projects. In fact, they can be used in place of sub-projects, allowing you to reduce complex project hierarchy. Optionally, if you prefer, you can transfer your sub-projects into task groups
If project hierarchy is working for you, it is not necessary to transition existing sub-projects into task groups. However, if you prefer, you can structure any future projects using task groups instead of sub-projects. For more information, see Create Task Groups article.
If you want to transition your sub-projects into task groups, you can do so by adding all of the tasks within the sub-projects to task groups and then moving those task groups to the parent/master project.
Identify or create the master project that will serve as the place where all of the tasks from the sub-projects will go. This could be the parent project in the hierarchy or it could be a completely new project.
Organize tasks within sub-projects into task groups. This step is necessary in order to quickly move the tasks into the master project. For this purpose, it might be best to create a single task group in every sub-project and then place all of the project's tasks into it. For more information, see: Create Task Groups, Move Tasks Between Task Groups.
Move the task groups into the master project. After you put all of the tasks from the sub-projects into task groups, you can move those task groups into the master project. You can read more about the process in the Move Tasks Between Projects article.
Delete empty sub-projects. After you move all of the task groups from the sub-projects into the new master project, all of the empty sub-projects can be deleted.