port -f upgrade <port> rebuilds already installed stuff

[Lorin Rivers]

I recall that using port -f upgrade outdated would do this (reinstall
stuff that was not outdated) but that, until today, performing the
same command on an individual port would only install what was
out-of-date. Today, however, when I upgrade the few ports that were
outdated, lots of stuff was rebuilt, some of it for every port I
upgraded.

What changed to cause this? I'm pretty sure this is new.