EI/IAG is bound to enter into an SPA with, amongst others, non oneworld airlines which provide or will provide a non-stop service between Dublin and Chicago (line item (570) iv above). This means United (the only non-oneworld airline that currently provides non-stop service between DUB and ORD).
As such, for as long as UA operate DUB-ORD (and/or LHR-ORD, covered in line item (570) (i) above) and for as long as UA want to maintain the PSA, Aer Lingus/IAG cannot stop/drop it.
EI is free to co-operate with AA, or anyone else they choose, but they are not free to "drop UA for AA" as you put it in your question.
Only UA can decide to terminate the current arrangement. For EI/IAG to unilaterally terminate the existing arrangements they had when they were an independent carrier would be uncompetitive, and therefore such decisions have been taken out of the hands of EI/IAG and instead placed in the relevant partners' hands.
Last edited by irishguy28; Sep 29, 2016 at 11:05 am