I have made a few post on how blizzard could do this without screwing up the lore. Basicly we take the 2 1h swords to the guy that made them in shadowlands. He takes them and reforges them into Frostmourne.
We then could be given an option in the weapon forge for our artifact where we could hit a button and go from 1h swords to 2h Frostmorne.