Previous Thread - http://www.gamesas.com/topic/1435018-quick-questions-quick-answers-the-32nd/

Carried over from the last thread, so it doesn't get lost:

Say I want to expand a dungeon, and I need to add a new (interior) entrance. Is it safe to mark the original door initially disabled, but leave it persistent?

You might need to change its form ID - I'm not sure. I'd say you'll have to test it. Visit the dungeon before you make the change so your saved game has the state of the door. Leave and then visit it again after you've made your change and make sure the door is disabled.