I was helping one of my fellow bus passengers switch from GNOME to Plasma (long story) and this tripped him up: the only way to see the Alternatives pop-up is from the context menu that appears when you're NOT in panel edit mode. When you ARE in panel edit mode, the hover pop-up shows items for configuring and removing a widget, but not replacing it with an alternative widget. We should expose that there too.
Actually I see now that if you right-click, it is visible while in panel edit mode. It should still be available via the little hover pop-up though.
Now it is!