How do we know this is "already"? I think this picture shows the very last moments before Megs get back to life. The Constructicons fitted Megatron with the new legs and the new cannon, now they are smacking the final pulses and spark/s into Megatron, which is what makes him shimmer and shake.
This also means that the Protoforms in the desert are NOT Constructicons in their Cybertron-form, since they already have earth-modes this early in the film, there is no real point in changing back when they get to the desert, and then change yet again.










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