There are a few servos where the centre section is glued to the motor and electronics. Trying to change that is the best way to needing a new one!
If you can find some older servos, like Futaba's 131 and 132 series then you could play around with motors and gears as they will come apart completely.
With the price of a servo, and the high chance (in my case!) that fiddling with them will end in tears, it's best to buy the servo that does what you need. Check this out...
http://www.servodatabase.com/servos/all
...for all the data you need on the vast majority of servos out there. HTH
