I guess it depends on whether multiclassing feats and paragon path multiclassing count as retraining.
PHB p.28 says, under Retraining:
"When your class table tells you to replace a power you know with a different power of a higher level, that doesn’t count as retraining—you can still retrain an additional feat, power, or skill as normal."
Thus, I would think, any replacement other than what you're told to do by your class table, does fall under the Retraining rules, including multiclassing.
It goes on:
"Power: You can replace a power with another power of the same type, of the same level or lower, and from the same class..."
Paragon Path Multiclassing says:
"11th level, you can choose to replace one of your at-will powers with an at-will power from your second class."
That is a specific exception to the part of the rule that retraining can only retrain from the same class, but it doesn't let you bypass the type and level limits.
Basic Attack has no level, thus you can't exchange it for any power that does have a level.
PHB p.28 says, under Retraining:
"When your class table tells you to replace a power you know with a different power of a higher level, that doesn’t count as retraining—you can still retrain an additional feat, power, or skill as normal."
Thus, I would think, any replacement other than what you're told to do by your class table, does fall under the Retraining rules, including multiclassing.
It goes on:
"Power: You can replace a power with another power of the same type, of the same level or lower, and from the same class..."
Paragon Path Multiclassing says:
"11th level, you can choose to replace one of your at-will powers with an at-will power from your second class."
That is a specific exception to the part of the rule that retraining can only retrain from the same class, but it doesn't let you bypass the type and level limits.
Basic Attack has no level, thus you can't exchange it for any power that does have a level.