Kelim, Chapter Twenty, Mishnah Four
1) If a large trough was so damaged that it could no longer hold pomegranates and he fixed it to be used as a seat:
a) Rabbi Akiva says that it is susceptible to uncleanness,
b) But the sages say that it remains clean unless its rough parts have been smoothed.
2) If it was turned into a feeding bowl for cattle, even if it was fixed to a wall, it is susceptible to uncleanness.
Section one: According to Rabbi Akiva, although the trough cannot be used for its original purpose, since he set it up to be used as a seat, it is susceptible to impurity.
The other sages basically agree with Rabbi Akiva, but add the caveat that the trough is not susceptible unless its rough section has been smoothed to make it fit for a seat.
Section two: If he turned it into a feeding bowl, then it is susceptible to impurity even though it is attached to the wall. The vessel is not considered as nullified due to its attachment to something that is attached to the ground.