Why are the people you have known for years more fun to work with than newbies? Most newbies I know (cabin and cockpit alike) are very enthusiastic about their work. I think it is great to see new people taking the opportunity to learn new things. One shouldn't be doing the same job for too long, better to move on after a couple of years.
What pilots learn in a conversion course? Mainly handling the new bird, getting acquainted with all systems and normal/abnormal procedures. This is true for any conversion course, regardless of the size of the airplane (thus, a conversion from a 744 to a 737 is in general not less demanding than a conversion from a 737 to a 744). Of course, after the conversion course, the practical difference is mainly the length and pace of the flights and the variety of destinations -- again for cabin and cockpit alike.