- Only secondary stats can be changed, meaning no switchin up the base attributes like intellect or stamina.
- You can only switch one thing, and it can't be changed to something that the item does not already have, so if you have hit on the piece you're changing, you can't put more hit from another stat.
- There's a fixed amount decreased from the initial stat and added to the new stat you're applying, based on a flat 40%.
- The item you're stat changing has to be ilvl 200 or above.
With the changes to talents and the removal of ALL our hit talents, you're going to find yourself in need of help there most likely, especially if you're arcane or frost. You'll find yourself with the choice of gemming hit (which is now blue btw), or reforging hit from other stats (my first choice would be crit in most cases). Depending on the amount you're reforging, you should take a look at the amount you'd be getting from the gem or from the reforge and compare it to what you'll need, especially in the case of hit. I'll even go so far to say that being a tiny bit under hit cap isn't so bad, especially if you regularly run with a boomkin or a shadow priest (or a draenei). Keep in mind that you'll also be able to forge in the valuable Mastery stat once this patch goes in, which will give you a number of other bonuses based on your spec. Until Cataclysm and the new professions hit, this is apparently the only way to acquire the stat, since none of the gear will likely come with it pre-forging. (From the look of the beta info that's out, Fractured will be the name for Mastery gems instead of armor pen, which is going away, and they will be red.)
So if you don't need hit, what do your change your stats to? Well, intellect is replacing spellpower, but it's more than likely your gear is going to have that on it anyway, so that's not going to be an option. Most spell casters have a haste soft cap to reach for, but it's more ambiguous with mages - it's very unlikely you'll reduce your arcane blast all the way down to a second, so fairly often haste is a stat of slightly less or equal value to spell power (Int in the case of cata) and is a good choice if your piece does not already have any. Crit doesn't scale as well with arcane as it does with fire and frost, but it's still better than being over hit cap and isn't a bad addition to the pool. If your gear already has all these stats, you can just as easily add some mastery for those yummy bonuses we discussed.
For the shaman, I suspect all our gear is going to have a slap of spirit on it, and we'll probably rarely see another piece with hit, with the exception of elemental tier gear for the set bonuses. On one hand, this is great because that's only one set of gear we'll need when we change specs! On the other hand, elemental and restoration don't always stack the same things. While haste is an excellent stat for both specs, the soft cap is different between them and elemental CAN still benefit from being over cap. However, with elemental, you still want spellpower, now intellect, over haste. This deals more with gemming than reforging or enchanting, but it's certainly something to pay attention to if you're sharing gear. With all the Mp5 changing to Spirit when the stats change over, I suspect elemental shaman will find themselves drowning in hit, so that'll be the go-to stat for reforging if you have it anywhere on your gear as a secondary stat. Please not that spirit cannot be reforged. You're stuck with it. Like mages, intellect and haste are the top two stats once cap is reached and crit comes in behind.
By no means am I saying to take mastery last after all the other stats; in fact, due to the bonuses, it'll probably be the first thing you put on your gear once the patch drops since there's no other source. Since all the bonuses are increased by a percentage, there will likely be a mastery cap, though it'll be pretty high I imagine.
When you get a chance to reforge, play with the stats a little. It costs the amount you could sell the item for to change it, each time. However, if you screw up and decide you don't like what you did even after it's done, you can restore the item to it's original unforged stats for free.