Upgrade time is fairly low and will likely go down. However, things can change overnight in the regional market. These are what I call "growth upgrades." People are moving up because the company is growing, not because people are leaving. Chasing the growth upgrade, in my opinion, is risky because by the time the 2 years or less are up, the growth may be done, or the company may even be shrinking or bought out!

If you're completely concerned about upgrade first and foremost, I'd chase an "attrition upgrade." When people are leaving a place in droves, that is likely to continue, with only minor slowing if the market takes a dump. Why? No one wants to stay at such airlines. However, such airlines will make you want to chuck the whole career by the wayside on occasion...

Just my own personal opinion and take on it...some stuff to think about, good luck.