Thanks QF009 for your quick response.
Two years ago I bought an AONE3 ex CAI from AA and I was allowed (without challenge) to select the first flight of seven in North America to be the 'purchased' one. I bought an L class segment and was able to upgrade a few days before I used it. So, two years ago it was apparently possible to do what I now want to do.
I think this is another example of an extra rule that AA has invented which is outiside of the published OneWorld rules.