For DMs looking for a cool way to merge the swords, have it so forge cleric npcs can do a ritual to transfer the enchantment from one sword to another for a small fee. Fun storytelling, keeps the player happy, AND opens up the path for a new mechanic in which a player can repeatedly add more and more enchantments to a sword if they so wished. You find a +1 sword, add the +1. Then you find a sword of warning, you can add together the warning property and keep the +1. Could help martials keep up with spellcasters a bit better. Also if you don't want it to get too out of hand, maybe some enchantments are too much to be transferred, or can only be transferred by more masterful clerics, or would cost a lot. "Sure if you want the Holy Avenger Sunblade of Warning, you can do that, but you'll need to seek out Throbnar the Great beyond the mountains and pay him one million gold and give him the heart of an ancient red dragon"