You can change your DN name or any other field in your profile (which you can edit) to your liking. My suggestion is to put DN name which you usually prefer and use more often because DN name in your profile tells people how to contact you in DN for a duel and if you put there an account you don't use that much, there is no point in it.
For necroposting, if you want to post in a topic which is not yours, then try to stick at least within one month approx but if you think you need to post even in an older topic for example "your favorite ygo characters". You want to post what you think but topic got a bit old. You can still post there since its considered like a timeless topic where you can reply at any time you wish. But if you reply in a topic like "march 2012 banlist predictions" like few weeks after banlist got revealed, thats of course necro posting.
As for your dorm room or any other topic which you created, you can post in it when you wish to, especially dorm room. Double posting is prohibited but not if 24hrs passed from your last post. So, if you wish to post again in your topic you should wait at least one day if you were the last one to post in it.
Now you got a better insight.