Integrating the quests would seem like the harder route to me.
Create a "new" quest in the new quest engine with the same criteria. Then remove the ability to get the old quest. The old quest system is still active and anyone with those quests active would still be able to keep and complete them, but no new quests under the old system could be offered.
While I believe it would require some work and testing to build the "new" quest, I would think it would be fairly straight forward since the old quest provides the blueprint.
The old quest does provide the blueprint for the basic content, but there is still the issue of implementing in within the newer engine. That probably wouldn't be too much of an issue, but the old system sill needs to be removed. We couldn't just leave the old one active, given that now we introduce the problem of having simultaneous quest systems trying to work on the same game objects (The NPCs, which we don't want to duplicate).
