1) Is the bottom part of the switch identical for all switches and only the springs and stems are the difference?
2) To replace the switches from red to brown for example do i have to desolder the red switches and solder in the brown ones or can open up the reds and switch out the red springs and red stems for brown springs and brown stems.
1) Yes the switch tops and bottoms are the same for all MX swithes. It's the springs/stems that make the difference.
2) It depends.
If you have a Plate Mounted board, that doesn't have the switch opening cutouts, you'll have to desolder, swap, and resolder. This is the standard procedure for most stock/commercial keyboards with a plate. I can't think of a stock plate mounted keyboard that has the switch opening cutouts on them but one might exist.
If it's got a custom plate with the cutouts or PCB mounted, you can just open the switch up and swap it.