"sale and/or assignment" would definitely include a sub-lease.

From Black's Law Dictinary:

assignment

noun. the act of transferring an interest in property or some right (such as contract benefits) to another.

I think this 717 transaction falls under the AirTran contract language.
We better demand a definitive answer from management on this question of pilots coming with the airplanes. The fact that they have been silent on that subject is very troubling.
Think about it - they would save a bunch of money on training if they brought the pilots over.