The ability to "convert" saves up to a higher version would save a lot of bugs
regarding "missing assets" not present in the save.

I may be able to, I'd just have to read the save format.

Thoughts?

I know I might be bumping this and devs might get angry in receiving a new
(rather useless) email, but wasn't this shipped in already? 

After all, some old savegames versions now works and says that it was converted...

commit a9c5044c78cb3ce11a3fdb22e6ccd0f2346fa3d4
Author: fluzz <fluzz@gmx.com>
Date:   Wed Mar 2 18:31:49 2016 +0100

    Introduction of a framework for savegame converters, and use it to
    fix a savegame bug in the 0.16 version..
    
    Those converters, composed of piped filter functions, can be
    used to fix bugs in savegames, or to convert from an old version
    to a new one, provided that the changes are not too complex.
    Do not expect this framework to be able to easily resolve our
    well known 'savegame incompatibility' issue...
    
    This framework is used, in this commit, to fix a bug in savegames
    from the 0.16 version, when German language is used (Lua error
    when parsing the .sav file). See savegame/filters_0_16.c for
    more information about that bug.

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I'm closing this for now... But if there is something I forgot or which requires
attention do not hesitate to re-open this, though.