I would be surprised if they simply hire the herd of regional airline FAs. Normally, I see a real difference in customer service competence between mainline DL staff and those on the contract carriers. (I'm not in a position to comment on the relative safety and emergency capabilities).
This will be an important hiring class for Delta. The last number I saw put the mean age of flight attendants in the US at 48. So, this new group will represent the future of cabin service. Maybe Delta is like me - I'd rather hire and train new minds instead of breaking habits and reforming people with other experiences.