It's all up to Yahoo. They basically said that there is no specific criteria that they use to determine whether to change a player's position or not.
Personally, I wouldn't count on them changing it any time soon. Josh Smith owners have been begging Yahoo for him to be SF/PF eligible instead of just PF since he started a ton of games as a SF. And Yahoo just recently changed it (I believe it was less than 1 week ago). That was after I read probably over 50 posts on here similiar to yours about Smith, and lots of them said they had complained to Yahoo.
So if it took them that long to change a pretty well known player like J-smooth, dont hold up hope for STevenson.
Here is something I copied and pasted from another post on this forum:
* All position eligibility requests are reviewed, compiled, and forwarded to our data provider who makes the final decision on position eligibility.
* Because Yahoo! is not responsible for modifying position eligibility, we cannot speculate on future changes or comment on the decisions about individual players.
* There are no set criteria (minimum number of games, minutes at a new position, etc.) for position eligibility. A new position will be added when our data provider determines that a player's primary position has changed.