In all Arma games so far the player will stop completely when choosing to change weapons (from say a rifle to the pistol)
This has a few negative side effects like the game feeling a bit clunky. Especially when going from a swimming to walking stance since the player will freeze completely to get his rifle making the transition feel weird. Other similar situations include ejecting from a vehicle or going prone with a rocket launcher.
By allowing the player to move while changing weapon the experience of such situations will feel much smoother and the game will feel more polished. Just like when we got the ability to reload mags while moving.
To avoid run and gun situations you could make the player move much slower while switching weapons. The important part is that the player can still move so we don't get that "stuck in animation" feeling