On the one hand, I agree that the 40th anniversary, SS86 version of the brand's main bad guy SHOULD be a new mold.I would be *very* shocked if an SS86 Megatron is a warmed over remold.
SS86 Prime isn’t and I’d expect nothing less for Megatron.
On the other hand, Hasbro and Takara's repeated reusage of the Siege mold seems to indicate that someone in a position to make the decisions considers it to be the definitive not-a-gun-mode G1 Megatron. So, I would not be very shocked if an SS86 Megatron either used similar engineering or was a partial.
On the other, other hand, Megatron's biggest moments in the movie are fighting Prime in robot mode, and getting turned into Galvatron. So, maybe a G1 Megatron that just transforms into Galvatron (with a tiny Walther pistol accessory for his minions to wield) is the best call for SS86 Megatron?