This started a few years ago when I tried to articulate a flip n change Lockdown using bionicle joints. Didn't get anywhere apart from cutting it up so dropped it.
Revisited it last week with a junker ROTF Jetfire and gave up on it transforming, so alas it's an Action Master.
I cut apart the backpack and lowered it. Head, torso, shoulders and feet are Lockdowns. Lower arms and legs are Jetfires.
By a happy coincidence, the screw holes on Jetfire's legs were just about the right size for the posts that used to connect Lockdown's front and back, so I've used those to connect his legs.
I reversde Jetfire's legs so Lockdown could stand up straight, and used Jetfire's feet to fill out his shins.
This started as a 'just mess around and see what happens' but the end result pleasantly surprised me.