Mike Bloomberg is now officially in the race. How does this change 2020 for the Democrats?

Not clear that it changes it at all. Bloomberg's bet is that he can spend tens of millions of dollars between now and Super Tuesday, on March 3rd, and get a bunch of votes and a bunch of delegates on that day. If that happens, it's bad for Joe Biden, it's bad for Pete Buttigieg, because he takes votes away from the moderates. It would help Warren and Sanders. But he could also fall completely flat and be out of the race after Super Tuesday.