itch.io

Ok so I did a couple of tests and figured out two methods for letting the player change their character's outfit! 

The first and simpler method uses expression files and regular Ren'Py choice menus. Once you've created expression files for each outfit (in my case, I called them uniform1, uniform2, and uniform3) and added them to the PreppyCustomSprite folder in your Ren'Py project's images directory, you can add a couple of choice menus to your script whenever it's time for a costume change. 

This first menu allows the player to pick an outfit.

label uniform_select:  
    
    menu:         
        "What will you wear today?"   
       
        "Skirt and button down":            
            jump u1         
        "Skirt and sweater":             
            jump u2         
        "Vest and slacks":             
            jump u3

These other three are basically just an "are you sure?" choice that lets the player preview the outfit before confirming their pick.

label u1:     
    show player uniform1      
    
    menu:         
        "Are you sure you'd like to choose this outfit?"
        
        "Yes":             
            jump next_scene         
        "No":             
            show player -uniform1             
            jump uniform_select  
label u2:     
    show player uniform2      
    menu:         
        "Are you sure you'd like to choose this outfit?"
        "Yes":             
            jump next_scene
        "No":             
            show player -uniform2             
            jump uniform_select  
label u3:     
    show player uniform3 
     
    menu:         
        "Are you sure you'd like to choose this outfit?"
        
        "Yes":             
            jump next_scene         
        "No":             
            show player -uniform3             
            jump uniform_select

This seems to work well if you just want to let the player pick between a couple of predetermined outfits.

If you'd like to give the player control over the details of their appearance with a separate character customization screen, (e.g. mixing and matching different shirts and bottoms, selecting colors, etc.) you're probably going to have to go the blend_parameter route. This method is a bit too complicated for me to explain in one comment, but I can whip up a little demo project if you'd like to see what I've figured out so far! I'll probably keep messing around with it in the coming weeks :)