i recall in ICC we could do this however i also recall with cata at the start they decided to make it to where you could not enable heroic mode after killing a boss on normal mode for that lockout.
im currently in a group claiming that is no longer the case and we can switch between the two without restriction as long as someone is qualified to enable heroic.
i just did normal 11/12 10 man and zone out to turn on into 25 man normal then went inside and i turned it on heroic 11/12 heroic 25 man. It works just fine with me.
The only restriction I'm aware of is you can't engage the last boss on heroic unless the previous bosses have been killed on heroic, you can certainly engage any other boss on heroic even if the prior bosses were killed on normal though.