Hello !
I need a little clarification about one point :
"Create individual profiles for each movie in these Box Sets. Use individual UPCs if they are available or use Disc ID (read on a DVD-ROM) if not. Create each of these individual profiles in line with the standard Contribution Rules with one exception - Cover Images. If a film is individually packaged, use the cover images from that packaging."

So I have a 2 DVD set in a normal DVD case, only one UPC for both. Like the Robocop Trilogy for example.

The movies are in the database with their own UPC, because they have a "solo" edition.
What should I do ?

1 - Just add the previous profiles to the boxset, without changing anything.
2 - Changing the UPC to the DiscID for each movie and adding them to the boxset, changing the cover by the parent cover.
Or a third solution ?

You should first try to add the child profiles via "By Inserted Disc". If the Disc ID is already stored in an existing profile, it's very likely that the content is the same so that you won't have to change much of the data before resubmitting it as Disc ID profile. But you should check the profile against your disc anyhow, like Pete already wrote.

If I am understanding you right... then yes. If there is a set that are not  in their own case (with it's own UPC) then it should only have disc id child profiles.