So you normally use a Soldier, but found a great spot for a turret. Problem is, you're completely leveled up, and everything is in your soldier class. Too bad for you, you have to make a new character, just for that one spot. There is, however, a way around that.
Go to your abilities section and select "Sell All". You will be bumped down a level, but you get all the points you spent for redistributing.
Say you distributed 20 points before selling them all, you'll notice you get all 20. Meaning when you level up again, you will then have 21 points for distributing. Now you can put those 20 points into whatever you'd like (excluding the rank 5 options) and you can put that one point towards a light turret. If you continue to level up to 20, sell all, level up, sell all, you'll notice those points just pile up. Doing this, you can have an unlimited supply of points. Takes forever, but you can have every single character choice (Soldier, Medic, etc.) at max level (20, or 24). Again, it takes a long time, but it really pays off, especially in online games.
If after you reach Level 20 / Rank 5 and spend 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.