We’d like to let you change between normal/heroic per-boss in both ToC and ICC, but we’re not ready to commit to that yet. We’ll know more as ToC gets closer and we have more time to look at how its built, and how hard it would be to change.
All I’m confirming in this post is that you definitely won’t be able to do both difficulties in the same week, and that you definitely CAN do 10 and 25-player versions in the same week, since that was previously something we were considering doing differently.