Your True Self is infinite and cannot be changed. That being said, your physical character can always be changed at any given moment.
If you wake up and decide that you want to be a professional runner, you can take actions towards being that. You can research what is involved, start working out everyday, and eating healthy. You can join running groups and start training for marathons. Eventually, even the most sedentary person can be a fitness inspiration. These are all decisions that your character can make to reinvent itself. During this whole process, your True Self has not changed but it will always guide you towards positive outcomes.