I created a project with several milestones. After a few weeks, it turned out that it would have been smarter to use subprojects for some of these milestones because these milestones should have milestones or subprojects themselves. Therefore, I would like to convert some of these milestones into subprojects.

A few weeks ago, I created a project which also has a few milestones. I want to convert some of the milestones into subprojects.
(why 1x) The milestones should have milestones and subprojects themselves (which I realized just now), therefore I would like to convert the milestones into subprojects.
(why 2x) Milestones themselves are not allowed to have subprojects as stated in the Project user guide, therefore I would like to convert them into subproject.
(why 3x) For simplifying the project itself, I want to distribute the tasks among several subproject, but in the beginning that was not clear, therefore I created a milestone.
(why 4x) The milestone was designed for collecting the work for one master student, but the project grew very fast and therefore I want to distribute the tasks to different subprojects and milestones.
(why 5x) I could also archive the milestone create a new subproject and assign new tags to the tasks from the milestone, but this would take quite much time because the milestones contains lots of tasks. Additionally, I would like to keep the name of the milestone.

I hope this is enough information for the description of the root problem.