Old tool in slot 1.

Craft a new tool in crafting bench. Hover over the finished tool box, click 1.

Expected behavior: New tool takes the position of old tool, in slot 1. (Very handy when upgrading tools, etc, and it has up until 15w44b always worked like that).

What really happens: Nothing.
The new tool can only be placed in an empty slot.

(There are a lot of similar bug reports, but nothing that I can find, which specifically addresses this issue.)