RE: transission to 2.4 Ghz
I don't know what the manufacturers are going to do but I can tell you that if you're buying new now you may as well go 2.4, when you decide to get another one in 6 months or whatever then you are going to have to replace any recievers as well so you may as well just start out that way.
Yes you can mix and match servos, there are some old airtronics servos that are wired differently but anything within the last few years is all compatable. RCKen has a nice picture he posts with the color coding of the various servos though.