Some preliminary thoughts:
- Pendragon works because it restricts what starting players can be. This gives it focus. (Lack of focus is what makes D&D such a great game, but it is what made the original Changeling: The Dreaming so difficult to play. This is a reflection of the special nature of trad-fantasy games and D&D in particular, a point which Zak Smith makes very well here. D&D works when you have a lot of options because everybody shares a broad understanding of what it is; in Changeling having all those options and a free reign but no real direction made the thing a bitch to make an actual game out of.) Therefore, Changeling Pendragon should be about young fae (preferably sidhe) in the service of a noble house on the cusp of knighthood, and go from there. This makes the game manageable, taut, directed.
- Pendragon's Traits map almost perfectly to the idea of the Seelie and Unseelie (and legacies) in Changeling. You would barely need to change the system at all. In fact the Pendragon system of Traits is probably a better fit for Changeling than the original rules.
- The winter phase fits fine with Changeling's emphasis on the importance of seasons. Again, you would barely need to change the system at all.
- Glamour, Willpower and Banality can all work using the Passions system as their basis.
- The major problem is cantrips and magic, which don't really fit readily into a given Pendragon mechanic; indeed "magic is not the point of a Pendragon game". I'd be tempted to either a) just lift the Changeling system wholesale or b) adopt the Pendragon approach and basically make it an NPC ability.
I would also have to remove all the tedious references to "gothic-punk".