Basically, it's like GM Connected from NHL 14, however, the entire league is simmed to certain points.
For example, it'll be simmed from the fantasy draft, then like a week later for preparations, to the mid season mark, then to the playoffs, and finally, the off-season, and the cycle repeats. So it sims quite fast.
And yes, teams get to change line ups during the season, I would send rosters out to GM after the fantasy draft.
Sadly, there are no open teams as of now, as all 31 teams have been taken.