My son turned 13 start of this last fall and moved from a 31"22oz 2 5/8 to a 30" 27oz BBCOR bat. My thinking was adding less weight was more important than losing an inch in length. He's long-armed and lanky so reach wasn't an issue. Hasn't had the magic juice from puberty yet so no the strongest kid at his age. He didn't have much problem with the transition other than being a little behind of faster pitching which I would expect. Considering he's also adjusting to 60' distance vs the 50' that would have something to do with it as well. While hitting off tee at home he uses a 28OZ wood bat and also uses some 29oz skils bats for hitting small wiffle balls.
I didn't invest a lot in this in between bat this fall, $65 new in wrapper from local shop b/c I knew I would get him another bat after this fall which now he is using a 31" 28oz during travel team hitting.
Whether right or wrong I have no idea. I think the main thing is training with a heavier bat to get stronger but then also training with the bat you expect to use in games. I don't think the reduced length had a negative effect but considering the change in distance mound-plate, I wouldn't know how to quantify and distinguish those changes.