Here's my good buddy's career progression at UA:

1) Flight Engineer on the DC-10

2) First Officer on the DC-10

3) Captain on the B737-300/500

4) Captain on the B757-200/B767-300 (common type rating).

And he has made this career progression for one important reason: more money.

BTW, he is now scheduled to begin flying PMCO B767-400 aircraft once the merger is fully accomplished.