Like... say... Replace Kyle with Luke and Luke with Kyle?
Here is the way:
Get Pakscape. (This is to replace Luke with Kyle)
Open assets1.pk3, and go into models folder.
Then go into the Players folder.
Then go into the Luke folder.
Copy the contents into a folder on you're desktop.
Now, make a new PK3 file. Inside it make a folder called models, and inside that make a folder called players.
Inside that make a folder called Kyle.
Then copy the contents that you copied to you're desktop into the Kyle folder.
Now, save the .pk3 file as Kyle_luke.pk3.
Try it ingame, and it should work. Other people do it with NPC files. I dont.
You might as well switch the model path in the .npc files.
No, just singleplayer
Well, then, there is an easier way. Just open the .npc file from assets1.pk3 in the ext_data folder and edit Kyle's model path to Luke's, and vice versa.
Open it up in Pakscape, and take a peek. Edit it all you need.
Note: .NPC files can be opened in Notepad.