Most importantly, such changes will not take place overnight. I do not recall the transition period for TWA, but I would expect something as complex as AA and US, with different reservation systems, etc to take a year or more.

In the unlkely event of a merger, it is reasonable to expect the Aadvantage program to survive, along with the Citibank affiliation.

I assume most Barclays US cardholders have done so for the mileage churn, little risk in at least one more cycle.