Originally Posted by
sosafan
As u2g0d said, yes they do.
Just to give a data point where this mattered, I had used up my US Airways miles, and then the few miles left had expired. But when the two airlines merged, I couldn't get my US Air miles added to my American lifetime miles.
The same thing happened to me - I even had an old statement with the US Airways number, my password, etc. I lost those lifetime miles in the merger (which probably weren't many to be fair).