You would have to have first hand knowledge of both lots. If there is nothing out of the ordinary, then feel free to mix them SO LONG as it truly is RL-15 to RL-15 and VARGET to VARGET and so on. Age can have somthing to do with it as the longer a type is manufactured probably the bigger the change. If lots are 50 years apart, make sure that one doesnt burn much hotter than the other.
IE if your old lot gives you premium results at 51.6 grains, make sure that 51.6 grains of the new lot isnt making too much pressure.