Copa's application process is a pain, believe me I went though it, I had to get a Panama CPL and the processs is slow and nobody really knows how it is supposed to work, after a week in Panama and missing work in the US I finally called it quits and came back, I have an interview with a regional in a couple of weeks.

If you can get on with Copa and they can do the conversion for you then go for it, otherwise, stay in the US. Also dont expect excellent pay or travel benefits, and the employee treatment is better in the US than in Panama. And upgrade will take FOREVER.