Change player into model onentered?

SHORT ANSWER: Upon entering the server, you could redefine the Character property in the Player instance.

LONG ANSWER: There are some guidelines you may need to follow to make a functional Character object.

SCRIPT

• You need a server script with .PlayerAdded and .CharacterAdded, because this will be the fundamental element to transform the Character object.

-- Ideally, you should put this in game.ServerScriptService.
game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        -- This is where the magic happens.
    end)
end)

• I've seen scripts where it does the transformation itself (i.e. re-positions and re-sizes the body parts for you), but I find it easier to make a base model for the Character object first and store it somewhere (ideally, game.ServerStorage).

CHARACTER

• As mentioned before, store the Character object in a storage instance (skip this if you're just going to create a script for the transformation). Anchor all of the parts.

• In order to sustain the life of your Character object, a head/torso MUST be connected (Motor6D) and a HumanoidRootPart MUST be present. Refer to the default Robloxian Character object and see where the Motor6D joint objects connect to.

• In my experience, keeping a Humanoid object in the Character object in Studio never worked out, as I keep dying. So in your script, create a new Humanoid object onto the Character.

Now all you have to do is make a spawn point for your character (with CFrame), rename the Character object every time it spawns, and then you're pretty much done.

This is how you redefine a Character object, add the Humanoid object in, and rename your character:

-- If your Character object is in game.ServerStorage
newCharacter = game.ServerStorage.YourCharacterObject:Clone()
newCharacter.Parent = workspace
newCharacter.Name = Player.Name
Player.Character = newCharacter

Humanoid = Instance.new("Humanoid", newCharacter)

newCharacter.Archivable = false