First, the checkers should ideally have two sides, like with some checker sets that use a star for a Checker King. That way, you can flip the checker (or "promote" it, so to speak) when it makes a complete tour around the board.
Second, if a promoted checker gets captured, it would make more sense to keep it promoted, since otherwise, this condition is too unlikely to happen.
If a checker makes a complete turn it gets removed from the board. There is no need for promotion and double sides. And a checker that exits the game can no longer be captured.
If a checker makes a complete turn it gets removed from the board. There is no need for promotion and double sides. And a checker that exits the game can no longer be captured.