This feature could be implemented as in phpMyVisites (it works well)
It makes it easy and secure to update from one version to another.

The software checks that the version in the database is the same as the version in the version.php.

If the version.php has a newer version, we update the database with all the changes that were done between both versions.

We can also execute some random php code as every version provides a file containing the changes to apply. This way of doing things is easy and very modular (you can do what you want during the update).

Display version number
Task also include display the version number somewhere in the admin UI, in the META tags on all piwik generated pages, an in the error/exception messages.

To do by default in first piwik release that has update feature
when updating from previous to first version with update system we need to:
- create indices now created at install: see comment below
- purge old archive tables containing temporary archive data
- remove cookie detection as it is completely wrong
- remove color_depth field
- alter table piwik_site and remove the update CURRENT_TIME