As I understand it (as I'm in a similar position): You're entitled to your oneworld status benefits when flying oneworld. You're generally entitled to your AAdvantage benefits when flying AA marketed and operated flights and using your AAdvantage number (as well as allowed benefits when flying Alaska). Though they may overlook booking MCE with your AA FFN and not penalize you with seat loss (unless there are changes) if you change the accruing FFN, you're technically not going to use AAdvantage status benefits if you change your FFN to another airline's. You might retain a benefit, but you're taking a risk. E.g. Entering your EY FFN and burning a SWU?