I put this under "heavy/scratch" because there was a lot more work involved than what I was expecting.
So yeah, simple torso swap? Not even close.
I found out that the torso, pelvis and shoulder hinges are completely different on both molds. They may look the same, but engineering-wise, they're quite different.

This came about because my ER Megatron had a number of parts that were yellowing badly and it annoyed me. I have way too many Siege ones, and due to not liking the paint on the Netflix version, I decided to sacrifice it to make a better Megatron. I also had a junker ER Optimus that donated his hands for the project.

First, a run down of the mold differences:
Torso- There is a.long pin that runs through the upper chest that holds the backpack in place. The inside of the Siege chest has supports for this pin. The ER version doesn't, and has a slightly different shape all together.
Shoulder hinges- This was a surprise. The square-ish pieces the actual shoulder hinge is mounted to is different on the ER mold. The little posts that slot into the chest are positioned differently, and there is sculpted detailing that won't allow for a straight swap between Siege and ER. Because I wanted to use the ER chest and Siege shoulder mounts, I needed to carve a bit of detailing away and reshape the posts that hold the sections in place.
Pelvis- The mushroom peg is located differently between the two molds, so in alt mode, the legs sit slightly higher. Not a big deal, but still odd.

I'm leaving the colors how they are. The chest is lighter, but with the darker grays of the rest of it I think it looks pretty darn good in hand.

Other stuff done:

• Painted the head Marvel black.
• Painted the sides of the torso and biceps red.
• Carved off the stock hands and replaced them (within the back hand shields) with ER Optimus hands. More work than was necessary, but I like articulated hands.
• Removed all the battle damage from the Siege tank parts.

I really want to find some waterslide transfers of the chest swirls, so if anyone knows of any...

Without further ado, I present my hybrid Megatron:

 

I just prefer the ER headsculpt. The Siege one is more lifeless in my eyes.

 

I don't have the figure with me, but I'll try to explain.
If you remove the swing panel that attaches to the back, you need to cut off or sand down the sculpted detailing that is supposed to represent grenade launchers. They keep the arm mount panels from folding in the entire way to seal the chest up.
Those shoulder mount panels also need the tab that fits into the sides of the chest trimmed a bit. The holes they lovknin to are different than on the Siege chest.

As for the hands, I essentially just cut the stock hands off and hollowed out the backs enough to fit the new hands in. Took some time to get the depth right that would allow the new hands to fir in the spot they fold into, but it wasn't difficult.

MP Sideswipe hands may not fit. I had a spare set of the KFC ones I had thought of using and they were too big.

 

The shoulder mounts (not the hinge joint holding the arm on) are from Siege, which is why I had to trim the post slightly.
I'll double check this later, just to be sure.

Unfortunately, a Dremel is almost a requirement for bigger cutting and sanding work. I have one of the bigger professional ones, but in all honesty, a cheap Tough Stuff one is just as good for small work.

The gun is the one from the Centurian pack, but with a few centimeters cut out of the barrel where it meets the gun body to shorten it. The stock length is way too long.

 

Yeah, we're talking about the same piece.. I used the Netflix mold, which is darker and has the red paint:
Megatron - Transformers Toys - TFW2005

 

To answer any questions on the hands, the wrists don't move. With the hand having a somewhat thin base after cutting enough away to make it fit in alt mode, I decided to not bother with rotation. Probably could have, but it wasn't as high a priority as just making the Prime hands work.

Also, added some printed ankles that allow the feet to fold up in alt mode:

 

I was considering scanning the ones from the MP and doing the ink jet thing.
Just need to buy an ink jet printer...