I'm pretty sure you were dreaming. Hero and Legendary ranks are semi-permanent (usually permanent but can be reverted if an admin sees), and at least usually last for a month.
Your enthusiasm intrigues me. You have dreams of earning a blue half-coin? :-/
I think admin can demote any rank why you say Here and Legendary is semi-permanent? Same applies to each and every rank I guess.
He's talking about a different a different matter
, there is a bug where demotion for Newbies, Hero member and Legendary members is not handled properly by the forum. See examples where accounts with less than 400 posts still have hero member status. Sure admins have the power to delete most of the posts and demote a rank or two, but I haven't heard of such cases. Anyway, this is quite OT
Do a little research.
I read some thread before that once you reached the legendary rank and you delete most of your posts but your activity will not be affected and you will remain as a legendary but I never thought that hero member has the same feature also or just a bug?
It's exactly this.
Furthermore, a quote from theymos confirms this:
IIRC only Newbie, Hero, and Legendary members don't get demoted normally. Maybe I'll fix it at some point, but it's not really a big deal.