I imagine that on some level you can modify your stats to imitate other classes, but ultimately, I think it's going to be the abillities that truly define the classes. I could build a Titan that is agile and has low shields but a high Discipline to mimic the Hunter, but without a Cloaking Abillity, it just isn't the same. Perhaps there will be a mode to swap abillities, but I don't imagine it coming up very often in standard play.