Originally Posted by
80ktsClamp
Given the wording of the press release, I'd say it's most plausible that the 321s are mostly replacements and the 333s are mostly growth.
Here is what the press release states:
The 30 A321s will expand Delta's existing fleet of 126 A320-family domestic aircraft. Delta's first three A321s are scheduled for delivery in the first quarter of 2016, with 12 more due in that year. The remaining 15 jets are scheduled for 2017.