On 7/7/2017 at 1:25 PM, mahri726 said:
The entire model is in public domain. However, on Blendswap, the model is classified as fanart, being based on a workart.
As Tom wrote, the best option is to get permission. Keep the documentation.
The second best is to verify the license is valid, or at least appears valid, such as the website URL, a copy of the license file from the web site, and a screen capture in case the site or content goes away eventually.
In both cases, keep a copy with the original assets themselves. You don't want to be in a position where you believe it was under a free license but was actually mis-licensed. If you are later accused of infringement but can pull up documentation that it was obtained under a different license from another site that can help you out in the long run.
I've seen projects that keep several directory trees for assets and their permissions. One directory tree for original or ones they built, another for content that was purchased, another for content that was obtained online for free or from other sources. Everything they didn't build themselves had links to the documentation. They could also be sub-categorized by license.
On 7/7/2017 at 1:25 PM, mahri726 said:
if I download the model, then alter the hair so it would be less similar to the one in the workart, could I use it in my game without having to ask for permission ? ... Does the copyrighted elements of an derivative work affect/infect the rest, like a GPL code or each element is treated separately ?
Yes. The people who create the first work have the legal right to control derivative works as well, including revenue from the derivative work. This is in several areas of law, not just copyright.
Some people mistakenly believe there is a specific amount of change, such as 10% of the work, that suddenly the derivative becomes non-derivative, but that is wrong. It is entirely up to a judge. Judges have found that even basing a fan work on another product, such as placing the world in the Star Wars universe, was enough to be a derivative work.
There are many other potential concerns with what you describe. One is finding the actual level of risk, another is finding the potential costs if that risk happens. A good lawyer can walk you through those. Legal defense is expensive, a single legal claim can bankrupt a person or small business. A lawyer up front can help you avoid most issues.