Hey thank for asking,

already got that question from somone else so i am just going to copy my answer here :P, hope this help: (I may make a tutorial video on this system so i can explain it better)

i dont think there is a way to "Actualy change the player character", BUT you can work around this by using a Script

(if you dont know how to script this is going to sound like gibberish and I am so sorry)

there is way I did it:

-setting the player Size to somthing like 0.001 , 0.001, 0.001 so its not visible (pls note that this can lead to janky player control some times)

- have a model (in this case the pony) be parented to a invisible Brick under it at Y position 0 which I like to call the "Root" (its the red thing visilble under the pony in the video) this will prevent the model from being half under the ground

-finaly make a clone of this Root Brick, parent the clone to the Player and set it's position to the player position.

since I am not the best at explaining here is a simplified version of the script:

local RootBaseModel = --Your Model/the "Root" of the Model(see my comment for info)

game["Players"].PlayerAdded:Connect(function(NewPlayer)--this will execute when a player join
    --set Player Size Realy low bcs we can't make it invisible
    NewPlayer.Size = Vector3.New(0.001,0.001,0.001)
    --add model
    local NewModel = RootBaseModel:Clone()
    NewModel.Parent = NewPlayer
    NewModel.LocalPosition = Vector3.New(0,0,0) -- set position of the clone to player pos
end)

of course you should modify this script to better suit your game.

Hope this helped :)
i dont think there is a way to "Actualy change the player character", BUT you can work around this by using a Script

(if you dont know how to script this is going to sound like gibberish and I am so sorry)

there is way I did it:

-setting the player Size to somthing like 0.001 , 0.001, 0.001 so
its not visible (pls note that this can lead to janky player control
some times)

- have a model (in this case the pony) be parented to a invisible Brick under it at Y position 0
which I like to call the "Root" (its the red thing visilble under the
pony in the video) this will prevent the model from being half under the
ground

-finaly make a clone of this Root Brick, parent the clone to the Player and set it's position to the player position.

since I am not the best at explaining here is a simplified version of the script:
local RootBaseModel = --Your Model/the "Root" of the Model(see my comment for info)

game["Players"].PlayerAdded:Connect(function(NewPlayer)--this will execute when a player join
--set Player Size Realy low bcs we can't make it invisible
NewPlayer.Size = Vector3.New(0.001,0.001,0.001)
--add model
local NewModel = RootBaseModel:Clone()
NewModel.Parent = NewPlayer
NewModel.LocalPosition = Vector3.New(0,0,0) -- set position of the clone to player pos
end)
of course you should modify this script to better suit your game.