If you pick the same weapon group with Weapon Legend as you did for Fighter Weapon Mastery, your proficiency level goes up to Legendary, but does that mean that Fighter Weapon Mastery essentially ceases to exist?
Sure you could pick a different group and have legenary in one and master in another, but that might not make much sense for a character story arc involving focusing in a single weapon.
Is there any indication that, should you choose the same group, you get to retrain Fighter Weapon Mastery? If not, why not?
The proper word is not "negates" but "subsumes." Weapon Legend first brings all your profs up to where Weapon Mastery brought one group. Then it brings any one group farther. Eventually Versatile Legend subsumes Weapon Legend by bringing all your other profs up to match that group.
So retraining Weapon Mastery once you have Weapon Legend, or retraining Weapon Legend once you have Versatile Legend, would do nothing.
So retraining Weapon Mastery once you have Weapon Legend, or retraining Weapon Legend once you have Versatile Legend, would do nothing.