POuring powder from one container into another, particularly into a can is a good( bad?) way to break granules down into fines, and that can change the burning characteristics of the powder over time.
My brother an I screened a can of powder inherited from our father that Dad had used out of the can, using one of those pouring spouts that is soldered to a can lid. He did very little field shooting, and didn't want to bother buying a powder horn. So, this powder got shaken back and forth in that can , and Dad was want to do an inventory of his unopened cans, and turn them up and down to see if they were full, not trusting his memory. Anyway, the full can we screened was FFg, and we got 1 oz. of FFFg powder, and almost 3 oz. of FFFFg powder. That left only 12 oz. of FFg powder in an unopened can of powder.
My brother was not happy, until we screened other cans that we had bought- some years ago- and found very little fines in them. We screened the powder from a powder horn, and found a bit higher percentage of fines than in a new can of powder, but not a significant amount. We discussed our father's loading habit, and his inventory practices, and decided that the higher percentage of fines we got out of his can of powder probably relates to how he handed it after purchase, and not to anything the company did back then. We did learn a lesson about pouring powder back into the can, for no reason at all.
As long as your horn is kept in a cool, dry place, you should have no problems storing powder for decades in that horn. I was given a powder horn a friend found in an antique sale, and it still had powder in it. There was no way to age the powder, but the horn was extremely dusty, and dirty and had not been handled or used in years. The powder burned just fine.
