It all came down to operations, cost, and schedule. Renovating the concourse allowed them to keep part of it open instead of demolishing the whole thing, freeing up more gates for use during construction. That also allowed them to keep the ramp control tower in that concourse in operation the whole time. The concourse was in decent shape, so renovations would reduce costs, especially since there would be less need to tear up and replace concrete and asphalt on the tarmac, among the most expensive and time-consuming parts of the project. And speaking of time-consuming, it obviously sped up the schedule -- the whole East Side Reconfiguration project should be done at least a year sooner than initially planned.

Probably most interestingly, the change gave Delta more operational flexibility -- the re-evaluation document [PDF] notes that Delta will be able to operate its A321s out of 9 gates in the 60s concourse, rather than just 1 in the original proposal.