Sperzel locking tuners
i really don't know how much these cost, mine were a gift , check stewmac.com or warmoth.com. you should have no problem changing tunings because of the tuners, but i find that the strings get 'used' to a tuning and dont response very well to alternating (that's a really subjective opinion btw).
installing is really easy: they don't have a screw, but a a single pin that prevents rotation, since the pressure that holds it to the headstock comes from the hex in front of it. a lot of newer guitars have this system, an if they dont, all it takes is a little drilling, which doesn't have to be perfect since the tuner covers the hole.
if your guitar has the old screw system, the only problem would be the ugly holes left.
if you use the trem a lot, they don't do as good as a locking system, but with subtle or no bar use (like me) they're great