Doesn't exchanging weapons in 5th happen during the action phase? That means that you couldn't attack then switch weapons to do an off-hand attack, as the attack completes the action. I don't think it makes sense to be able to use a melee set action and then just switch to ranged to use the hand crossbow bonus action attack, unless the melee is a one handed weapon and the hand crossbow is being dual wielded in the off-hand.
