Also a "downgrade" is an update.
It is just a mental difference.
As also time flows forward only, also updates flow only forward.
If you want to have the concept of a "downgrade", simply add another package to the f2 update site that transforms the "more recent" Installation to a "later" one.
From the theroetical point of view there can be a update and a "downdate".
In the physical realization there is only a update. This conceptual simplification eliminates various tricky issues that arise with rollback and revert while at the same time it does not Limit the capabilities.