[Bug 372487] Re: kmail downgrade wipes dimap cache

[Harald Sitter]

As I said, it is a matter of resources. If we had enough human and
computing resource we would support up and downgrading from all
supported Ubuntu version to all other supported Ubuntu versions. But we
don't have that.

It might sound incredibly easy to implement a simple data check, but
that simple data check depends on humans to tell the checker between
which versions an incompability was introduced. Then again humans make
errors, so you need a sophisticated computing system to back that up and
either run daily up and downgrade checks and then somehow evaluate the
results to again poke a human who has to validate that indeed there is
an incompability and that the computing system (which was also created
by a human) didn't mess up, or the computing system is indeed
intelligent enough to evaluate the source code itself to trace changes
that could cause incompability, still using the above documented chain
of triggered human check-ups though.So, that is what needs to be done on
an application level, but most applications use libraries which are
shared across multiple applications and those applications might do
completely different things with the same set of libraries, so you'd
need to duplicate and adapt the process for each application, each
library, simply put, each part of the software stack independently.

This is just a very raw picture of what would be needed to ensure
downgradability, in fact some stuff goes far beyond what can be
automized without an artificial intelligence, so in order to ensure the
whole Ubuntu stack works, you probably need to lock all of northern
america away for a month and let them test everything manually.

Amongst other reasons, this is why almost no software project, company
or single developer even claims to support downgrades, long ago the
industries faced the fact that it is pretty much impossible and just
declared it a standard that there is no support for downgrades.

And that is why a network/system administrator will always do test
upgrades (clone the data and try it out), ask google for known issues
etc. etc. and also why any home user should backup their data,
especially before upgrades.

That said, a data backup is one of the means to recover from a mistake
:P So, if the Ubuntu upgrade would ask you and assist you in doing an
backup, that would be very good (and in the long run that will happen).

Anyway, as a general rule: downgrades from 4.y to 4.x are never compatible, there will always be at least one component that changed the config layout or the file format etc.
Down from 4.x.y to 4.x.x  is usually a pretty save bet, but might also cause issues, very unlikely though. 
For Ubuntu it is never save to downgrade, there are too many things involved in the software stack, so you are bound to experience breakage at some point (also with each Ubuntu release you get a new KDE, so the first rule I mentioned will always apply).

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kmail downgrade wipes dimap cache
https://bugs.launchpad.net/bugs/372487
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