If I were you I’d:
book a decent refundable (points? Booking.com?) hotel in PDX for the duration of the stay as per the current ticket. Perhaps with two (split) bookings to limit my liability if I decide to change plans within 24hrs of the stay commencing.
call BA after I arrive in PDX and make the desired change at a price more acceptable than right now…
If that fails enjoy a couple of days in PDX as per hotel booking(s) and crack on with the rest of the trip.
good luck!