Yes, you can change your list at any time. Sometimes people list games they end up simply not liking, others have other stuff come up in their life and have to pick shorter games, and others have started up a game not on their list that they end up really liking so they swap it onto their list since they know they'll go through it.
As noted above, the only thing I suggest people don't do is replace their incomplete games with already completed games at the end just to get more completed games in their row. The idea really is to set goals for backlog games and complete them. But some wiggle room is alright.