Well, I have no idea how they will choose to implement this, but I think it would be foolish for them to set it so that you sync as soon as you join a lower level character's party. Would make it very weird if you want to, say, craft while the party was forming up. "Oh, sorry, suddenly you're only a level 15 Armorsmith, so you can't make that level 35 recipe while you wait." I have to think it would be something that you voluntarily initiate.

Although, I can certainly see that it might automatically cap you when you walk into a dungeon. I'm not sure how I feel about that. Maybe they could have some that do and some that don't. That would be different than level-syncing though, from what I understand.