Going to lay it straight for you since the answer to this question is buried in the other FFXIII thread:
The game is split up into 13 'chapters' of major plot events.
You spend 10 of these chapters on a rail killing bad guys and defeating bosses, earning your summons and figuring out how the 6 player classes work together along the way.
You may not:
1) switch party members until most of the way through Chapter 10, nor may you level any member's jobs outside of their prescribed 3 until more or less the same point in the game
2) undertake the optional missions (read: bosses) until you beat Chapter 10
3) level your weapons past their second forms until you more or less clear chapter 13 and double back (you get portals back to the open field).
4) Ride chocobos until you've cleared the first 7-ish missions in chapter 11.
In Chapter 11 you get on a space whale and fly to the moon. No not really but that's the scale of the change of venue you see. You're free to switch out party members, do side missions, and by now you'll have aquired all of the summons the game has to offer. There are some very difficult missions that you probably won't be able to clear until you've beaten the final boss (which unlocks the final tier of character abilities/spells/etc). You can level up your characters a lot in chapter 11 but you can't truly max them out until after the end of the game, making the final 10-ish 'hard' missions doable. Clear all the missions and...nothing, congratulations there's nothing in the game you can't kill, go farm dark matters and trapezoihedrons to convert all your equipment to their ultimate forms.
The answer to your second question is yes, there are varying weapon designs, but upgrades along a single weapon 'line' dont' change their form until the ultimate weapon is unlocked. All told there are 5 distinct models for each character in your group, with textures differing to tell the various models apart. The only exception is Snow, who fights with his fists so his 'weapon' is really the design down the back of his coat which changes from upgrade to upgrade.
XI Wiki




