This is they way I have seen it play out in our world. First of all, just through actions, no matter how negative, a person of joylightlove Source energy cannot turn into dark Source energy. They have to have a huge commitment to becoming dark to the core of their being in order to make the "jump". Most humans are of joylightlove Source energy. Some are from other Sources.
Jumps between sources are possible. I have seen at least one being turn from absolute darkness to absolute light. Therefore the reverse is also, theoretically, true.
When a light being comes into the planet, often they will fall into the depth of sleepiness and/or despair. Many addicts, for example, are "fallen angels". Their actions hurt themselves and others, and they don't stop.
What this creates is a type of "crust" around them which is thick, heavy, and light cannot get through. It is made of fear, anger, regret and many open doors to low vibrational energies and beings. To all appearances, they are dark, but at their core they are light.
Remember though, that some of them are fully committed to becoming Power Over Others (POO) to the core. Don't try to save them!
Also remember that many human beings have had multiple incarnations on this planet while playing the light/dark battle of POO. Sometimes they "play" the bad guy, and sometimes they "play" the good guy.
Just because a person gets covered in POO, doesn't mean they can't then process it, dissolve it and shine bright again. But that can only be done when they decide to do it. You can't do it for them, and allowing them to smear you all over with POO is counter productive.
Some individuals don't have an emotional charge about being POO or Joylightlove. They can move from one to the other without effort or judgement. In this case, it's probable that they are completely done with the light/dark paradigm and ready to try a different Source energy and experience.