I finally upgraded MT 3.1.x to MT 3.2. The upgrade process was much easier than expected. The only error I encountered was sth like:
main([path to install]/mt/php/extlib/smarty/libs/core/core.load_plugins.php): failed to open stream: No such file or directory
which could be solved by simply rebuilding the whole site, as explained in
The new features of MT 3.2 are very welcome of course.