It depends on your strategy and wishigs. IMO,
do NOT change class of the Assassin!
Why? The assassin is the best fighter in the game - even a better one than a zenmaster - because the assassin can use Assassin's Dagger. Moreover, the assassin is a rogue, therefore, his lockpicking ability is fortified by the game. Also, the assassin can learn Moon Magic and become your teleportation guy. In the other words, Assassin belongs to the "final roles", i.e., to the roles which we take after collecting skills and traits by means of role changing. At any rate, you will LOSE when you change the assassin to the samurai.
As far as the Doublestrike trait and the 2nd Weapon skill are considered, both are guild level 4 (or level 5?) Warriors Guild skills available to everybody, including the assassin.