I am not an expert,but I would think that the down side would not knowing which tweeter to put in,possibly making matters worse. Try to find someone who has done exactly the same upgrade or do you homework big time.
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I think you'd be better trading the speakers for something else. If you mod them they'll be worthless on the used market so you'll really have to live with them, and as Drew pointed out it may not be a simple modification.
If the speaker is over bright then some extra drapes on the walls of the listening room can help to tame this also, since the room contributes to the tonal balance of the system.
You can get "removable adhesive" two-way tape and all types of colored felts from Michael's. Use two-way tape that wont leave a residue. Lowe's and Home Depot definitely sell a two-way tape that painters use. I forget what its called, but I remember it wont leave residue.
You can also buy the foam strips used to seal around window unit AC's. Just dont use its adhesive. It more than likely will leave a residue. Stick it to the two-way tape and let the tape do the adhering.