There is a function which swaps weapons (actually 3 of them), but it is private a can't be used by addons.
Lua Code:
OnSpecialMoveKeyPressed(SPECIAL_MOVE_INDEX_WEAPON_SWAP)
OnSpecialMoveKeyDown(SPECIAL_MOVE_INDEX_WEAPON_SWAP)
OnSpecialMoveKeyUp(SPECIAL_MOVE_INDEX_WEAPON_SWAP)