If Reeves finds a cast that he wants, but one that isn’t able to start shooting on the current schedule, there’s always the possibility things could change.
As far as we know, nobody has been cast for the upcoming film, especially a brand new Batman.
Alternatively, things may need to be pushed back if Reeves can’t find the cast he wants at all.
Of course, Reeves says that it is “looking like” the film may start shooting at the end of the year, which makes it sound like things aren’t quite set in stone yet.