Doing something like that by hand isn't very practical - to completely swap a character out with another, you have to edit every appearance of that character. Replacing each appearance is simple - just locate their character block inside of the map's person file (romfs/GameData/Person/(map cid).bin) and replace the asset id, face id, character name, and character description with those of the character you want to appear. You may want to change the class byte as well if you want the character to show up using their normal class.
That technique works for situations where the character is not recruited. Recruited characters don't rely on the character blocks in Person - instead, they are handled through the GameData file (romfs/GameData/GameData.bin). For swapping out a character, you really just need to worry about the map spawn file (romfs/GameData/Dispo/(map cid).bin). All you have to do for map spawn files is locate the spawn block for the character you want to replace and repoint the block's PID to the PID of the character you want to appear.
Making these kinds of edits isn't particularly hard, but characters often have lots of different appearances and IDs to track down in order to make a full swap. If you want to replace every character, you'll have to replace hundreds of blocks.
Anyways, good luck if you decide to try it. If you aren't really familiar with Fates' bin files,
this documentation can help you.