Hi,
I am using Digikam 2.5.0 and KDE 4.8.5 on Ubuntu Precise, but would like to work with the face recognition of Digikam 3.3.
Updating digikam to 3.3 on my system, requires a bunch of upgrades:
I would like to ask if this is safe to upgrade (most of the upgrading packages I don't know what they are good for, and if they would conflict with other Programs from KDE 4.8.5).

If not, would it be possible to start Digikam 3.3 on a live system, do the face recognition there and use the so-updated Database with Digikam 2.5.0 again? Is the database of the two versions compatible?

I suspect the version delta between Digikam 2.x and Digikam 3.x is too great for their databases to be compatible.
As for the screenshot, please be aware that Marble will also be updated to a newer version as well.

As kipi-plugins is also being upgraded, it is possible the functionality of other applications which use Kipi (Gwenview in particular) may be affected by this upgrade.