ATL: It was discussed elsewhere last year when the plans were finalized between the relevant airlines and the City of Atlanta. ATL will be extending the T concourse to the north and adding several additional gates to allow AA to consolidate its operations onto T. If I remember correctly, the understanding at the time was that United would shift down a few gates, making way for AA to pick up its additional gates (I believe 2-3) contiguous to its existing T9-T12, and then there would be another 1-2 gates left at the far (north) end of T that another airline would pick up (the consensus belief at the time was Alaska).
BOS: Massport has released multiple documents - compliance and regulatory documents, tenders, etc. - regarding its plans for Terminal B. Included in those documents was discussion about the planned consolidation - as long expected - of AA's operations on the LUS side of B, making way for Southwest to move into the current LAA side of B. As part of this consolidation, the LUS side of B will be getting a not-insignificant investment (I seem to remember it being several tens of millions) to expand and improve the security checkpoint, upgrade concessions, increase concourse/gate area footprints, etc.