I tried some other migration howtos, but none gave enough information to provide a flawless migration.
I had to google a lot and it took some time.
Maybe this howto isn't perfect too, but I'll try to update it!
If you have contributions / additions, please mail them to me (armin at xos dot net).
This is a HOWTO for a 2.4 to 2.6 migration.
This is the FAQ with the most common problems.
My experience and impressions.