Currently it's not possible to perform a bulk operation on a bunch of sub-tasks to move them to a new parent. This is rather annoying when splitting subtasks off from one task to another (e.g. from a "miscellaneous" task to a specific task once a set of sub-tasks gets large).
It would be great to be able to change the parents of any sub-tasks in a group when a move operation is specified. This seems to make sense anyways and would also allow moving subtasks (along with regular tasks!) to a new project if the new parent task was in the new project.