RigaMortus2
First Post
A basic attack is an at-will attack power that everyone possesses, regardless of class. The power comes in two forms: melee and ranged.
Paragon Multiclassing
If you have the Novice Power, Acolyte Power, and Adept Power feats for a class, you can choose to continue to gain powers from that class rather than take a paragon path. If you choose this option, you gain several benefits.
At 11th level, you can choose to replace one of your at-will powers with an at-will power from your second class.
Since Melee (or Ranged) Basic Attack is considered a power, it seems like you can retrain it with an At-Will from your paragon multiclass. Of course, one of the side-effects to this is, if you retrain the melee basic attack, you won't be able to take OAs, take advantage of the Warlord's Lead the Attack, and other similiar options. That's why I would recommend swapping Ranged Basic Attack if you can afford to.
Anyway, RAW this seems to work...