I managed to get "Tequatl's Hoard" and it contains a choice of one ascended weapon of the Sunless. If I understand correctly, I can change stats on almost any ascended weapon. Does this apply to this one also? Can I change stats on this weapon?
Take the weapon, the Exotic Inscription of the stats you want the weapon to be, 5 Globs of Ectoplasm, and an Anthology of Heroes, bought from Miyani or any Mystic Forge attendants to the Mystic Forge. This will change the stats of the weapon to the stats of the Exotic Inscription you used.
Note: The look of the weapon will change as well. Sunless is a dark green-ish hue, but if you switch it to be Berserker's stats, the weapon color will turn to Crimson. A simple transmutation in your hero panel will return the look, however.