I magnatised the arms on mine and adjusted his pose little.
If you're only planning on swapping one gun of the onto the left arm, i'd say cut up the multi-melta. I think it looks better on the left arm then the assault cannon.
YMMV.
Using a hobby saw, I cut on the plain round section just underneath the shoulder armour. I then glued on a piece of 1mm thick plasticard the same diameter to make up for bulk lost by the sawing.
Click on the picture to go to the zoomable gallery version if you want a closer look.
I basically did the same with the leg. Just remember to cut the correct leg for the desired pose. I cut the wrong leg and now he looks like he's taking an awkward step, rather then smoothly striding forward
For added dynamism, once you've cut a leg off, you actually have easy access to the waist joint, which can be modified in the exact same way as the arms/legs. I didn't think of doing that at the time and only realised after i'd glued him together.
So with a few cuts and a bit of plasticard, you can turn a very stiff looking model into one with a decent amount of movement.