Yes the agent is required to fix the schedule and not try to charge you extra. If necessary they need to ring their oneworld liaison desk to sort it out.

However this has the hallmark of a codesharing renumbering issue, and since it's not until May next year there could easily be 6 more scheduling changes between now and then. Personally unless there was something like a big festival or event around the time in question I would leave it until probably March next year to fix otherwise you'll only have to do it again and again. Moreover if this has just happened it may be that BA's rebooking process hasn't completed yet, it takes several days to recalculate everything, for reasons I haven't yet found out. But the bottom line is that you have paid BA to get you to Portland and that is what they must do by law. MCT at SEA is 90 minutes, so it's not that.