Hi Maryn thanks for your post - I'm happy to help! 

Currently it is not possible to manually reposition the Milestone shape's text box label to a different location. As a workaround I recommend deleting the default text box entirely (by simply clearing that text) then adding and positioning a separate text box manually where you need it.

However I certainly understand how this ability would impact your diagramming abilities! If you're willing our development team would love to hear more about this via our feature request form. Ideas submitted here are considered when prioritizing improvements to Lucidchart. Apologies for the inconvenience and thanks for your request!