[Caladbolg] No 2nd MF stat change?
You don’t change it in the forge. You change it by talking to the NPC in your home instance. It cost 100 unbound magic
The quest, itself, told you (in a bit of an indirect way) that these weapons would not be able to be used in the Mystic Forge.
You can talk to Ridhais, and exchange the weapon (to another, or another of the same, either with unselected stats) for 1000 Unbound Magic.
On the character that completed the story missions, go and find Ridhais in your home instance. She will allow you to exchange your current ascended weapon for a different one for 1000 Unbound Magic.
https://wiki.guildwars2.com/wiki/Ridhais
Good luck.
Hmm it was 1000? I guess I forgot a 0 :p
Also “completed” means after you have gotten the letter and went to talk to the pale tree not just finish making the weapon.