You're a little late to the party. They have beeen doing this for 5 weeks now. After today, or maybe after the 100% unlock, they should be opening it up to all the different classes so people can get any they missed. I also believe a blue post said it will be open a few days after release as well.
EDIT: I just looked and as of right now you can go back and do the previous classes.
i have also read the blue post about that and i recall something about having 2 weeks of time to do it, and rlly dont wanna miss em out. since i missed the DH and the barr, and yeah, u could already make the banners for every class when they were unlocked till now, but im talking about the sigils