Famitsu magazine revealed that Final Fantasy XIII will be putting a bit of a twist on the typical Final Fantasy weapon system. In Final Fantasy XIII you will be able to feed your weapons material which you can obtain through treasure chests, battles, shops, and by destroying other weapons in your possession. Materials will have a variety of effects on your weapons, but the main effect is that your weapon will gain experience points which are needed to level up your weapon. You can also increase and decrease EXP receive from materials by feeding certain materials to your weapons in a certain order.
Eventually your weapon will have a small star icon placed by it to indicate that its level cap has been reached. Once the cap is reached you have the option to change the form of your weapon which will increase its capabilities and give you new abilities, but you will need a special material to do this. There is a catch to upgrading weapons though. When you change the form of your weapon it will initially start out weaker then its previous level capped form, but eventually as you level the new weapon it will surpass the capped capabilities of your previous weapon.
While the weapon changes its form, the actual weapon class won’t change. For example if you are upgrading a rod type weapon you will get a different weapon, but it will still be in the rod weapon class.
Stores will still sell weapons, but there is still a clear emphasis on feeding your weapons. The weapons sold at stores will be at their base level, so they will be weak upon purchase until you build them up.
It sounds similar to Phantasy Star’s mag system to me, but it could be I’m just playing too much Phantasy Star Zero.