Avent Upgrade "Feasable" for F-35 Engine

Essentially, they are looking at adaptive versatile engine technology (Advent) technology, which could change engine bypass ratio in flight.

Interestingly enough, this was a part of the architecture for GE F120 engine, which lost to the P&W F119 in the F-22 raptor, from which the F-35's F136 engine was derived.