If I have a personality without the kensai keyword, who already has a weapon attached to him and I want to attach a Unparalleled Sword. Is this legal? I mean the Unparalleled Sword will give him kensai so it could work... If it doesn't work can I still choose which weapon I want him to have or am I forced to keep the old one or take the new one?
Unparalleled Sword and non-kensai
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
At the point the Unparalelled Sword is still in your hand, the Personality isn't a legal performer for Equipping a Weapon, even if it would as a result become legal. I see your logic, but Good Faith Rule doesn't go that far as to predict that the action will become legal ex post facto - it must be legal right before the Item enters play, which is when aspects of the game are accounted for. See algo Playing attachments, where determining restrictions on entering play (such as the Unique keyword, but also the Weapon keyword) are checked at Step C of the sequence. Your Personality wouldn't be Kensai until Step E, which is too late.
You also can't choose to "discard" one Weapon to attach another. The Personality must keep their Weapon unless an effect (explicitly) destroys or discards it, or better, transfers it (check Jungle Stockade for lots of fun toying with Weapons). Without such an effect, swords you Equip are sword you keep (pun pun )