Playing with the DB transfer, David has spotted some anomalies in one DB upgraded from 1.9 to 2.2:

• The backup_ids and backup_files tables (used by 1.9 backup) are present in an installed 2.2 version but missing in an upgraded one. It seems that they were deleted from both install and upgrade (a934bd23) but later re-introduced @ install.xml by (df997f84).
• ...

So, in order to provide everybody with proper upgrade from old versions we should:

1) Install old versions of Moodle (1.7, 1.8, 1.9 at least) and upgrade to 2.2 (in 1 or multiple steps).
2) Compare the resulting DB with a 2.2 new installed.
3) Take rid (always conditionally) of anything found.

Note that since 2.0, we are already doing that in automated way (2.0 => 2.1, 2.0 => 2.2... all combinations), so there is no any problem there just pre-2.0 sites can have a different schema.

Surely the changes will need to be applied both to 22_STABLE, 23_STABLE and master. Once and forget.

Ciao