Originally Posted by
EkekoBWI
Care of a speedy reply via Twitter, it's bad news bears all around:
1. Pay for bags at SEA and again at SFO; no ability to check through to GRU since it's 12+ hours between arrival and departure in SFO (there's no alternative SEA/SFO option on UA and inter-line would only add complications)
2. Upgrading a domestic leg will have no bearing on the international barrage allowance, even if part of the same ticket; SFO overnight is irrelevant to that allowance
Seem right?
Yes, that's pretty much what I expected.

I assume there's no alternative SEA-IAH service either?
Well, except that the SFO overnight is actually critical. If you were able to check the bag through, upgrading the domestic leg would definitely get you the higher allowance.