Bernie Sanders is definitely running for the Democratic nomination against Hillary Clinton. While I only see him winning the Vermont primary, he can potentially give Hillary Clinton a run for her money in the Iowa Caucuses and the New Hampshire primary by receiving 20-30% of the vote in them.
So you're thinking he will become a Democrat (last I checked, he wasn't) just for the sake of running against Hillary Clinton in the 2016 primaries?
From his public statements over the past nine months, that's exactly what it sounds like he's going to do (should he run). He says that he doesn't want to be a spoiler in the general election, and he's highlighted the fact that if you run in the primaries for either of the two major parties, that gives you an opening to participate in the primary debates. So that sounds like he's looking at running for president as a Democrat, but nothing's official yet.