nope. JFD ran into this ages ago, and both he and I tested various different methods of using this function or attempting work-arounds. None work. Deleting and re-creating the unit does not work because you cannot set a unit's "Level" via lua nor can you attach modifiers the unit may have had from being trained in a given city, for example. And because of the way combat XP works in civ6, from the testing I did back then any XP points you add to a unit via lua beyond that needed for the 1st promotion go *poof* once the unit selects such a 1st promotion.