he hasn't practiced at the position in over 3 years now. I think he could probably figure it out, but there's no denying that relearning the position would hurt his productivity and possibly even result in injury.
I've speculated about this before too, but at the moment Betts is providing far too much value right where he is for me to be excited about messing with him.
That said, if it worked the result would be a brilliantly productive second basemen, there was never anything wrong with his ability to field second base, he was pushed out of second by the existence of Pedroia who's not here right now. It absolutely COULD work, and the upside would be ENORMOUS considering what he can do with his bat.
Betts is one of the best offensive right fielders in the game, putting that kind of production at a middle infield position would be a massive advantage, But it wouldn't come without risk, and the team won't likely decide to take a risk of that nature.