Depending on the airline, it is not as simple in most cases as just accepting a new type and collecting your 4 stripes at the front office. There is a course to be passed, and history shows that a pilot who is new to a type and also on a command upgrade is vulnerable. Many years ago I had the choice of whether to fly the 737 or the Airbus and I took the Airbus - a choice I have never once regretted. I work for a fairly large company who once had all 737s and are now all Airbus. We have countless (as in many hundred) ex-737 pilots and I reckon around 1% of them would ever return to the 737 if given the choice. When you speak to guys who have done both, you hear the same comments again and again. The 737 had a better FMS, was better in a gusty crosswind and they hate the fact the thrust levers do not move. There is no doubt that crosswinds take a little time to get used to on the Airbus, but once you have the technique, it is just great. It was put very well to me that if you look at 737NG below the coaming it looks similar to the Airbus - if you look above your head the 737's overhead panel looks like something out of a Second World War German submarine! The cockpit is noisy, cramped, has tiny windows and is just a relic from a bygone age. There is hardly any room in the overhead lockers and it is just a poorer work environment for the cabin crew. It has been a great workhorse, particularly in the States, but its day is done. The technology available to aircraft designers now is so advanced (just look at the wondrous 787) but the 737 missed out on all that because of Southwest Airlines' need to maintain standardisation and common ratings between the Classic and NG. It was an opportunity lost - Boeing are capable of building wonderful aircraft, but their hands were tied on the 737 and it really shows.