> The absolute worst thing, though, was that changing a license should not be a minor (or a major) version number increase.

The license didn't change. It was always already GPL, due to the usage of GPL-licensed code, regardless of what the metadata said. The change just made the metadata correctly reflect reality.

[EDIT: I should clarify that technically mimemagic wasn't already GPL, but the only legal way to use it was by satisfying your obligations under the GPL, making it effectively GPL. The author did relicense his own code to be GPL instead of MIT.]

To me it seems like making your downstreams aware of that ASAP is pretty important, since this has important legal implications for them as well. Yanking the old versions and releasing an update with an incompatible version number is a way to do that, albeit one that's quite disruptive.