It sounds like you want the contents of a dropdown to update dynamically. While you *can* update a drop down contents via a data sync, it will not update to the screen without exiting the entry form and going back in.
I assume that is not what you are looking for though. As an alternative, I wonder if you couldn't use a data grid on the entry form. From the user perspective it would look and feel more like a list box than a drop down but it could work for what you need. The key option you would want to take advantage of related to the grid is the data binding settings on a given column. This allows a column of the selected row to be bound to a control on the form and that control would be assigned that column's selected value. Here is a link to a tutorial video that discusses the data grid
This *should* discuss the data binding option as I have described.
**This option of live dropdown updates is an open feature request on our side so we will be looking at it but in the meantime, I wonder if my suggestion above could work for you.
Thanks for the great question.