I am flying from SFO -> LHR June 2 and returning LHR -> LAX on June 17. I booked the return to LAX as it is a PE reward ticket and there was no availability into SFO for rewards.
Is it worthwhile to show up at the airport early on the 17th and see if I could get switched to the SFO flight? Currently the load check is showing 9's ...
You can certainly ask but I am not sure how successful you will be. It can depend on the loads but also your ticket type and how kind the check-in person wishes to be. You might be asked to pay a change fee and any difference between your fare and the fare to SFO. If you are on a very restrictive fare they may say its simply not possible but there is no harm in trying at all.
Just because load checks are showing 9s it does not necessarily mean that reward seats will be available. I would suggest that you keep on checking to see if any reward flights become available nearer the time and then perhaps try at the airport
I keep stalking the website to see if the seats open up to SFO or if rewards seats in Upper open up too.
Since it is a (U) reward fare I assume I would just pay the difference in the taxes on the fare and a change fee which is totally fine. But the ability to fly straight home would be worth it rather than having to deal with a short hop on Southwest.