There are a few restrictions:
- You can only change secondary stats (hit, crit, haste, mana regen, dodge, etc.).
- Only one stat may be changed.
- You can't change one stat into a different stat the item already has.
- You can't alter how much of the stat to change; it's always 40 percent(ish) of the original amount.
- The item must have an item level of at least 200 (level 80 epics or higher).
Where can you find these reforging guys? They are usually found in major cities near the enchanting trainers. If you can't find them, be sure to ask a guard, who'll point you in the right direction.
All in all, the reforging system is pretty simple and provides the opportunity for us to start trying out the new mastery stat without waiting for Cataclysm itself to be released. If you'd like to take a look at how this works, here's a gallery walking you the process through step by step.