You can use a pin to pop out the rod/peg piece that connects the elbows and shoulders in CoD/Halo figures, then swap them around. I've used that trick a few times. I've also noticed that even though Destiny figures have the same scale as CoD figures, the rods in their arms and shoulders actually are compatible with Halo figures.
Also, while the armour used on Spartans/Elites/Marines, etc. isn't compatible with CoD thighs, brute armour does fit easily onto CoD legs.