When you have reached Level 20 / Rank 5 and spent your Ability Points and THEN you sell the points to redo them you will LOSE A LEVEL! You will be put back to Level 19, Rank 4, and ALL of the XP for Level 20 is deleted. Once that happens it CANNOT be undone.
If you have a leveled-up player in Campaign mode you will get access to the more advanced abilities and other body types. But when you approach the Challenge modes the situation is the opposite, you do NOT want to have a leveled character because that makes the challenges worse.