What is the term for a word which has the meaning changed to its opposite?

Solution 1:

I can't believe I found this answer, but acceptable terms were coined in the 1960's- contranym and autantoym

The wikipedia article is title auto-antonym, which was coined at the same time.

Solution 2:

Queen Anne when looking at the beautiful new St. Paul's Cathedral, called it "awful, artificial and amusing".

Awful meaning awe inspiring.
Artificial meaning artistic.
Amusing meaning thought provoking.

The linguistic term for what happened to these words is pejoration. It specifically refers to when a term moves from being positive to negative over time, eventually losing its positive meaning entirely.

In linguistics there have been many attempts to categorize semantic shift, which is a much broader concept than merely definitions changing to the opposite. The closest terms that really refer to opposites though are probably amelioration and pejoration.