It is my understanding that you can also download mods that make mechanical changes to the world while retaining both copies (Vanilla and modded Crossworlds) at the same time. You can however, not edit the legacy campaign using the provided Editor. Unlike in other editors with an edit history, the modding done to campaign exist at the top level so they are permanent and cannot be reverted even while seperately stored you can have only one working copy to edit. This is to say that the Crossworld campaign encourage you to divert from the vanilla story arc and thereby content. The most important different is that you are free to edit the quest, dialog, and maps to your liking with the included editor.

The Cossworld campain is a seperate campain on the disk with sperficial differences as stated. The Armored Princess is vanilla game and also host game needed to install the so-called Crossworld Expansion <- do a search for this. As a old-timer with assorted copies AP, I think I can say more about that: No reason and a waste of time, but I played vanilla AP more. I modded my crossworld campaign and plays old AP to get the story.

King's Bounty: Crossworlds instaling