Replacing Back Case And Bezel

Hello. My name is John and i'm thinking changing the back black case of my iphone with a new, white one. I would like to know, if i can replace the back case alone and use the factory metal bezel (scratched as is), or i necessarily need to buy new bezel to complete the re-assembly? Another thing i would love to know is what parts (buttons, etc) can be moved from the old into the new case and what parts i need to replace with new (are irremovable). For example, the little protective grids at the bottom that cover the mic and speaker must be replaced or they are integrated in the dock connector and . That kind of details i would like to know in order not to mess the whole thing up. Thank you.