Look at it this way. It is a contractual arrangement based on a set of terms that existed when you bought it. Swiss wasn't a Oneworld partner when you bought it, so you can't use them at a later time. The codeshare stipulation is only one another Oneworld carrier, but that would be the carriers at the time of the contract. So you could change a codeshare from AA to LA on a LA flight (or vice versa), but you can't add airlines that were not part of the set of rules you agreed to when you bought your ticket.