This is a bit more generous than I'd use, but OK...
This is actually how it works, RAW, according to the DMG. Most people just don't realize it.
But this is a complete non-starter. In effect, what this means is that once a character has one magic weapon it has a bunch of them...just pick the one you need for the situation at hand. Bleah!
Why does it have "a bunch of them"? The magical properties are exchanged with another similar magic item. If you have a +1 bow and a Sunsword, you don't get a +1 Sunsword but a Sunbow and a +1 sword. If you want both properties, you still have to carry around both items. This just makes magic items less DM-dependent. If you find a magic sword, but you use axes, you can switch it out, making your axe magical and the sword normal. There is no power difference, just item choice.
Though the opportunities for thievery become brilliant! I'm a thief, I know (or suspect) you've got a +2 sword, and by the time you wake up in the morning I've got a +2 rapier and you've got a hunk of sharp metal. I then sell said +2 rapier and buy a mundane one, pocket the difference, and call it a night's good work. Lather, rinse, repeat until before long I'm stinkin' rich!![]()
Yeah, that's not the intent. I'll try to change the wording to clarify that.
"You may exchange magical or non-magical properties between two similar items you are attuned to by touching one to the other for the duration of a short rest. If an item doesn't normally require attunement, you may attune to it without using an attunement slot. For example, a +2 sword touched to a mundane bow would result in a mundane sword and a +2 bow."
Yeah, that's where this idea falls apart. What would be better is forcing some sort of choice at any level - OK, at 1st level I can use greatsword but gawds help me in tight quarters, or I can use dagger which is great for close-in work but kinda hopeless in the open field, or I can use a mace instead and bludgeon them instead of slicing them, or ...
Then at higher levels you can slowly fill in the gaps so by, say, 10th level you're up to 6 or 8 proficiencies and by 20th level you can pretty much use most normal weapons and maybe even a few exotics.
Creating other consequences to weapon choice would be interesting. That is my intent with this whole idea: to make weapon choice matter.