I'm sure there are lots of rules that are confusing and complicated (or maybe I'm just a simpleton) but thought this might be a good place to document them.

I'll start:

Recently bought a AMS-LHR-LAS-ORD-LHR-AMS flight in J. All on BA metal except the LAS-ORD which is on AA. The other week I got an email to tell me the LAS-ORD flight was now going to be leaving 2hrs 35 mins earlier than scheduled. That didn't fit in my plans so I asked the TA to put me on the next available AA flight, which is leaving in the afternoon.

Called BA yesterday to try and UUA the ORD-LHR leg to F and was told that I couldn't be upgraded as one of my legs had an AA flight number and not a BA one. To cut a long story short, the afternoon AA flight LAS-ORD is not a code share with BA therefore agent couldn't put in a BA flight number. Somehow this now affects the possibility to upgrade other legs of the journey that do have BA flight numbers and are on BA metal. I was told that the TA would need to change the flight number of the LAS-ORD flight even though the CS agent from BA admitted there wasn't an equivalent BA flight number for that particular flight....

The rules are the rules.....they just don't make sense to me....