pkgng vs random port moves

[J David]

Is there a best practice for what to do with pkgng when ports
maintainers randomly rename ports? (E.g. mail/p5-Mail-SpamAssassin
being moved to mail/spamassassin a few days ago without notice or
warning or documentation in UPDATING.)

Prior to pkgng, we used to use pkg_replace for this.  portmaster also
has an option for it.  But pkgng seems to assume that never happens
which is (unfortunately) not a valid assumption.

Thanks for any advice!