Flight coming up in the summer which has just been decimated by the expected AA schedule changes...

DUB-LHR: BA
LHR-JFK: BA
LGA-BOS: AA - cancelled
BOS-LAX: AA - now not possible due to canx LGA-BOS
--
LAX-DCA: AA - retimed 21 minutes earlier
DCA-JFK: AA with BA codeshare - codeshare has changed (does that constitute a cancellation?)
JFK-LHR: BA
LHR-AGP: BA

Is it possible to change the origin from DUB? Almost any airport in the UK is within 300miles. Although, obviously it's a different country, so not sure what the rules on that are? Also, I seem to remember reading somewhere (but cannot find any reference) that you can only change the gateway if that flight has been affected. Is that correct?

Ignoring the retiming of the LAX-DCA and the codeshare change of DCA-JFK, would the cancellation of LGA-BOS make me able to make modifications to the return as well?