Hi Kramer,
No worries, with so many options its hard to know what do to when you first show up. You really only have the first game or two to be able to change your character card/items if you aren't happy with it for free - after that it would cost your once ever "re-roll". Just bring your weapon cards to sign in, and ask that someone there help you rewrite your character, and they should exchange your weapon cards for you as well.
If you are asking how to justify these changes in roleplay ingame, its just a matter of suspension of disbelief. If you are basically planning to play the same character personality wise and just changing your weapon skills, this is no issue from a roleplay standpoint, just continue as if no change occurred and no one will mind. Continue to talk to the PCs you met last game as if you know them, remember everything that happened, etc. If you think you want to completely remake your character from the ground up, with a new personality, completely different goals and professions, etc etc, there is no rule that says you cannot remember everything from your first game, but in that case I would think it is probably best to start new, as you may find your new incarnation meshes better with other sets of characters (and it winds up being less confusing for everyone else!).
Hope this helps!
Defend the Land for it is the Base of the Pillar,
Elawyn Featherthorn Brightstar
Acolyte of the Circle of Ten & Chosen of Fey Lord Tillion
Guardian of Belladeen
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OOG: Kristen M.