reasons why downgrades are Not Supported
Package downgrades are not supported. Usually, they will work, since it is
often just replacement of a set of files with different ones (often the same
names) with different contents. I think if there were always true, downgrades
would be no problem (besides the general tendency for higher versions to have
more features and less bugs).
Two exceptions come to mind: Replaces, and maintscripts.
Replaces because of missing files in the partially-Replaced package when the
Replacing package is removed.
maintscripts often don't support downgrades just because downgrading has always
been advertised as "not guaranteed to work", so very few maintainers put the
additional effort and clutter into their code.
Are there any other reasons?