How do I revert a changed userId back to original userId?

Well, you can just create a NumberValue and put it in Folder in ServerStorage on Player join. You can name it as the player's name and its value a the player's UserId. And on player remove, you can delete that NumberValue. Here's an example, that you can use:

--//Functions
local function PlayerJoin(player)
	if game.ServerStorage:FindFirstChild(player.Name) then return end -- A double check 'if' statement whether the player name already existed in ServerStorage
	local NumValue = Instance.new("NumberValue") -- Creating a NumberValue
	NumValue.Parent = game.ServerStorage.Folder -- Parenting it to the Folder in Server Storage
	NumValue.Value = player.CharacterAppearanceId -- Setting its Value to the player's appearance ID
--	NumValue.Value = player.UserId  [If you really want to get the original player's character appearance, then get the player's UserId]
	NumValue.Name = player.Name -- Setting its Name to player's name
end

local function PlayerLeave(player)
	if game.ServerStorage.Folder:FindFirstChild(player.Name) then -- Checking whether the NumberValue have the player's name who is leaving
		local plr = game.ServerStorage.Folder:FindFirstChild(player.Name) -- Assigning a variable for easiness
		plr:Destroy() -- Deleting the Value
	end
end

--//Main
game.Players.PlayerAdded:Connect(PlayerJoin)
game.Players.PlayerRemoving:Connect(PlayerLeave)

And now you have a method to get the original player's UserId, then you can change the player's character to back to original.

Lemme know if it helps!