Your example made me think, I could perhaps just add Dovecot 2.0 as a new mail/dovecot2 package (also since dovecot 1.x is still supported). That will prevent issues with automated/unattended updates, and a warning MESSAGE will be sufficient. Seems ok, but really no one will want to have both installed, and they will CONFLICT. So starting dovecot-2.x via rc.d and having it upgrade park would still be great (I like your rc.d idea). That said, it's of course not fair of me to tell you to write the code.... Plus, a dovecot-2 with a warning and upgrade code later is still a big step forward.
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