We're basically back to how items used to work prior to 8, the only difference is that it doesn't require you to have an item active (circling or dragged behind you) before you can pick up another.
Double Dash was the only game that allowed you to swap since it had each character hold a different item.
I've played through 50cc and 100cc so far, and I have to say that I don't even understand why Nintendo added a second item slot. It rarely did me any good. More often than not, I'd have the item I needed to counter an attack that'd happen maybe ten seconds in the future, only for a ghost or a lightning bolt to cause me to lose said item. There's almost no point in holding onto items strategically, because they'll just end up getting lost or stolen immediately prior to when you need them. Until now, I don't think I've ever felt so strongly that a game's A.I. was operating solely to screw me over...even with past Mario Kart games.
I think Blur did Mario Kart better than Mario Kart in terms of item balance. It blows my mind that Nintendo's even calling this out as a new feature, given how uninspired it is. I mean, how hard is it to let players switch item slots and only have active items vulnerable to loss and theft? At least the ghost only steals the first item, but it blows my mind that the lighting bolt hits everyone aside from the person who uses it AND all of those players lose BOTH items.