Our party's Weaponmaster has been using a single handed weapon since level 1 and took the "one-handed" option. They have since found a Two-handed weapon. Is it possible to "retrain" the talent?
The retraining rules of DnD 4e only allow retraining powers, feats or skills. Class features outside those cannot be retrained. Unless the player of the Weaponmaster works it out with the GM, they're stuck with the choice and have to consider the advantage of the talent when choosing weapons to use in the future.
However, if the Weaponmaster happens to be a human, they can choose the Versatile Talent feat (from Martial Power 2) to extend the bonus to the other category of weapons as well.