I've been regrading quite extensively on AR, live AA NA and RU. I am going to pitch my 2 cents in.
Regrading high-end grades is only viable if you have the money to back it up. From my experience, it is much cheaper to regrade certain high-demand weapons (like shortspears, clubs, etc) than it is to buy them from players, because the items are so rarely for sale that the prices are ridiculous. People who have money WILL pay those prices because the items are highly sought after, so there isn't really a problem for players to sell them.
It is true that you may get lucky and get a Great Success on divine -> legendary weapon the first try, although consider that the same as winning a lottery where you have 1 chance out of about 23 for a Great Success. The number comes from my experience and is roughly universal for patch 3.0b across 3 environments I've regraded on over the past few years. So, be prepared to have at least 23 superior red charms along with 23 resplendent scrolls and 23 divine weapons to *somewhat* safely regrade a divine to a legendary. I say *somewhat* because you may not even get 1 legendary. On the other hand, you may get multiple of them, in which case welcome to the whale club. You can also offset some, if not all, of the cost of the above with whatever divines you regrade to epic and sell.
Never regrade anything divine and above without a charm and without a resplendent scroll. Silver charms have proven to work better, but the cost of them when used on a divine weapon is insanely not worth it, unless you are going for epic -> mythic.
Using celestial anchoring shards on celestial items (or divine shards on divine items) is only warranted if you are regrading hard-t0-get stuff like Red Dragon items or warrior medal armor. Don't bother using charms on regular obsidian celestial items but do use resplendent scrolls on celestials.
Craft and single-scroll-no-charm regrade obsidian items up until celestial.
Vika out.