I have booked a flight from LAX to SFO on UA for my uncle months ago. With some changes of plans, another segment from SFO to ORD or MDW is to be booked. My uncle has no status on UA and will check one piece of luggage, so he is going to pay $25 for the checked luggage from LAX to SFO.

Several questions:

1. If the new SFO-ORD segment is on UA, is it possible to transfer the checked luggage to the second flight when the first flight arrives at SFO? (There is no problem paying $25 again, but I just don't want my old uncle to do all the hassle.)
   
2. What if the new segment is on other airlines, say DL, AA or WN? Can the bag be transferred automatically as well? In case it is on DL or AA, how can the checked bag be paid for?

Thanks for your help!