For changes to both dates and subjects, the three options seem to be:
1) only the original works 2) only the new one works 3) the new one is canonical, and the old one redirects to then new one
Option 3 makes for the least breakage and failure, I think. I think option 2 is closest to how Wordpress does it (Wordpress was my model for the URL layout, and the one I'm most familiar with). Option 1 would be a very bad idea IMHO.
no subject
1) only the original works
2) only the new one works
3) the new one is canonical, and the old one redirects to then new one
Option 3 makes for the least breakage and failure, I think. I think option 2 is closest to how Wordpress does it (Wordpress was my model for the URL layout, and the one I'm most familiar with). Option 1 would be a very bad idea IMHO.